Moving picture coding method and moving picture decoding method

ABSTRACT

According to a picture coding method of the present invention, a coded picture identified by a picture number is stored, as a reference picture, into a storage unit; commands indicating correspondence between picture numbers and reference indices for designating reference pictures and coefficients used for generation of predictive images are generated; a reference picture being used when motion compensation is performed on a current block in a current picture to be coded is designated by a reference index; a predictive image is generated by performing linear prediction on a block being obtained by motion estimation within the designated reference picture, by use of a coefficient corresponding to the reference index; a coded image signal including a coded signal obtained by coding a prediction error being a difference between the current block in the current picture to be coded and the predictive image, the commands, the reference index and the coefficient is outputted. At that time, information indicating the maximum reference index value is coded and included into the coded image signal, and the commands indicating correspondence between at least one picture number and a plurality of reference indices are generated.

TECHNICAL FIELD

The present invention relates to a moving picture coding method and amoving picture decoding method, and particularly to a coding method anda decoding method using inter-picture prediction with reference topreviously coded pictures.

BACKGROUND ART

With the development of multimedia applications, it has become popularto handle integrally all kinds of media information such as video, audioand text. For that purpose, digitalization of all these kinds of mediaallows handling of them in an integral manner. However, since digitizedimages have an enormous amount of data, image information compressiontechniques are absolutely essential for storage and transmission of suchinformation. It is also important to standardize such compressiontechniques for interoperation of compressed image data. There existinternational standards for image compression techniques, such as H.261and H.263 standardized by International Telecommunication Union -Telecommunication Standardization Sector (ITU-T) and MPEG-1, MPEG-4 andothers standardized by International Organization for Standardization(ISO). ITU is now working for standardization of H.26L as the lateststandard for image coding.

In coding of moving pictures, in general, information amount iscompressed by reducing redundancies in both temporal and spatialdirections. Therefore, in inter-picture prediction coding, which aims atreducing the temporal redundancy, motion of a current picture isestimated on a block-by-block basis with reference to preceding orsubsequent pictures so as to create a predictive image, and thendifferential values between the obtained predictive images and thecurrent picture are coded.

Here, the term “picture” represents a single sheet of an image, and itrepresents a frame when used in a context of a progressive image,whereas it represents a frame or a field in a context of an interlacedimage. The interlaced image here is a single frame that is made up oftwo fields having different times respectively. In the process of codingand decoding the interlaced image, a single frame can be handled as aframe, as two fields, or as a frame structure or a field structure onevery block in the frame.

The following description will be given assuming that a picture is aframe in a progressive image, but the same description can be given evenassuming that a picture is a frame or a field in an interlaced image.

FIG. 30 is a diagram for explaining types of pictures and referencerelations between them.

A picture like a picture I1, which is intra-picture prediction codedwithout reference to any pictures, is referred to as an I-picture. Apicture like a picture P10, which is inter-picture prediction coded withreference to only one picture, is referred to as a P-picture. And apicture, which can be inter-picture prediction coded with reference totwo pictures at the same time, is referred to as a B-picture.

B-pictures, like pictures B6, B12 and B18, can refer to two pictureslocated in arbitrary temporal directions. Reference pictures can bedesignated on a block-by-block basis, on which motion is estimated, andthey are discriminated between a first reference picture which isdescribed earlier in a coded stream obtained by coding pictures and asecond reference picture which is described later in the coded stream.

However, it is required in order to code and decode the above picturesthat the reference pictures be already coded and decoded. FIGS. 31A and31B show examples of order of coding and decoding B-pictures. FIG. 31Ashows a display order of the pictures, and FIG. 31B shows a coding anddecoding order reordered from the display order as shown in FIG. 31A.These diagrams show that the pictures are reordered so that the pictureswhich are referred to by the pictures B3 and B6 are previously coded anddecoded.

A method for creating a predictive image in the case where theabove-mentioned B-picture is coded with reference to two pictures at thesame time will be explained in detail using FIG. 32. Note that apredictive image is created in decoding in exactly the same manner.

The picture B4 is a current B-picture to be coded, and blocks BL01 andBL02 are current blocks to be coded belonging to the current B-picture.Referring to a block BL11 belonging to the picture P2 as a firstreference picture and BL21 belonging to the picture P3 as a secondreference picture, a predictive image for the block BL01 is created.Similarly, referring to a block BL12 belonging to the picture P2 as afirst reference picture and a block BL22 belonging to the picture P1 asa second reference picture, a predictive image for the block BL02 iscreated (See Non-patent document 1).

FIG. 33 is a diagram for explaining a method for creating a predictiveimage for the current block to be coded BL01 using the referred twoblocks BL11 and BL21. The following explanation is made assuming herethat a size of each block is 4 by 4 pixels. Assuming that Q1(i) is apixel value of BL11, Q2(i) is a pixel value of BL21 and P(i) is a pixelvalue of the predictive image for the target BL01, the pixel value P(i)can be calculated by a linear prediction equation like the followingequation 1. “i” indicates the position of a pixel, and in this example,“i” has values of 0 to 15.P(i)=(w1×Q1(i)+w2×Q2(i))/pow(2, d)+c  Equation 1

-   -   (where pow(2, d) indicates the “d”th power of 2)    -   “w1”, “w2”, “c” and “d” are coefficients for performing linear        prediction, and these four coefficients are handled as one set        of weighting coefficients. This weighting coefficient set is        determined by a reference index designating a picture referred        to by each block. For example, four values of w1_(—)1, w2_(—)1,        c_(—)1 and d_(—)1 are used for BL01, and w1_(—)2, w2_(—)2,        c_(—)2 and d_(—)2 are used for BL02, respectively.

Next, reference indices designating reference pictures will be explainedwith reference to FIG. 34 and FIG. 35. A value referred to as a picturenumber, which increases one by one every time a picture is stored in amemory, is assigned to each picture. In other words, a picture numberwith a value added one to the maximum value of the existing picturenumbers is assigned to a newly stored picture. However, a referencepicture is not actually designated using this picture number, but usinga value referred to as a reference index which is defined separately.Indices indicating first reference pictures are referred to as firstreference indices, and indices indicating second reference pictures arereferred to as second reference indices, respectively.

FIG. 34 is a diagram for explaining a method for assigning two referenceindices to picture numbers. When there is a sequence of pictures orderedin display order, picture numbers are assigned in coding order. Commandsfor assigning the reference indices to the picture numbers are describedin a header of a slice that is a subdivision of a picture, as the unitof coding, and thus the assignment thereof are updated every time oneslice is coded. The command indicates the differential value between apicture number assigned a reference index currently and a picture numberassigned a reference index immediately before the current assignment, inseries by the number of reference indices.

Taking the first reference index in FIG. 34 as an example, since “−1” isgiven as a command first, 1 is subtracted from the picture number 16 ofthe current picture to be coded and thus the reference index 0 isassigned to the picture number 15. Next, since “−4” is given, 4 issubtracted from the picture number 15 and thus the reference index 1 isassigned to the picture number 11. The following reference indices areassigned to respective picture numbers in the same processing. The samegoes for the second reference indices.

FIG. 35 shows the result of the assignment of the reference indices. Thefirst reference indices and the second reference indices are assigned torespective picture numbers separately, but focusing attention to eachreference index, it is obvious that one reference index is assigned toone picture number.

Next, a method for determining weighting coefficient sets to be usedwill be explained with reference to FIG. 36 and FIG. 37.

A coded stream of one picture is made up of a picture common informationarea and a plurality of slice data areas. FIG. 36 shows a structure ofone slice data area among them. The slice data area is made up of aslice header area and a plurality of block data areas. As one example ofa block data area, block areas corresponding to BL01 and BL02 in FIG. 32are shown here.

“ref1” and “ref2” included in the block BL01 indicate the firstreference index and the second index indicating two reference picturesfor this block, respectively. In the slice header area, data (pset0,pset1, pset2, pset3 and pset4) for determining the weighting coefficientsets for the linear prediction are described for ref1 and ref2,respectively. FIG. 37 shows tables of the above-mentioned data includedin the slice header area as an example.

Each data indicated by an identifier “pset” has four values, w1, w25, cand d, and is structured so as to be directly referred to by the valuesof ref1 and ref2. Also, in the slice header area, a command sequenceidx_cmd1 and idx_cmd2 for assigning the reference indices to the picturenumbers are described.

Using ref1 and ref2 described in BL01 in FIG. 36, one set of weightingcoefficients is selected from the table for ref1 and another set of themis selected from the table for ref2. By performing linear prediction ofthe equation 1 using respective weighting coefficient sets, twopredictive images are generated. A desired predictive image can beobtained by averaging these two predictive images on a per-pixel basis.

In addition, there is a method for obtaining a predictive image using apredetermined fixed equation, unlike the above-mentioned method forgenerating a predictive image using a prediction equation obtained byweighting coefficient sets of linear prediction coefficients. In theformer method, in the case where a picture designated by a firstreference index appears later in display order than a picture designatedby a second reference index, the following equation 2a being a fixedequation composed of fixed coefficients is selected, and in other cases,the following equation 2b being a fixed equation composed of fixedcoefficients is selected, so as to generate a predictive image.P(i)=2×Q1(i)−Q2(i)  Equation 2aP(i)=(Q1(i)+Q2(i))/2  Equation 2b

As is obvious from the above, this method has the advantage that thereis no need to code and transmit the weighting coefficient sets to obtainthe predictive image because the prediction equation is fixed. Thismethod has another advantage that there is no need to code and transmita flag for designating the weighting coefficient sets of linearprediction coefficients because the fixed equation is selected based onthe positional relationship between pictures. In addition, this methodallows significant reduction of an amount of processing for linearprediction because of a simple formula for linear prediction.

(Nonpatent document 1)

ITU-T Rec. H.264 |ISO/IEC 14496-10 AVC

Joint Committee Draft (CD)

(2002-5-10)

(P.34 8.4.3 Re-Mapping of frame numbers indicator,

P.105 11.5 Prediction signal generation procedure)

In the method for creating a predictive image using weightingcoefficient sets based on the equation 1, since the number of commandsfor assigning reference indices to reference pictures is same as thenumber of the reference pictures, only one reference index is assignedto one reference picture, and thus the weighting coefficient sets usedfor linear prediction of the blocks referring to the same referencepicture have exactly the same values. There is no problem if imageschange uniformly in a picture as a whole, but there is a highpossibility that the optimum predictive image cannot be generated ifrespective images change differently. In addition, there is anotherproblem that the amount of processing for linear prediction increasesbecause the equation includes multiplications.

SUMMARY OF THE INVENTION

Against this backdrop, it is an object of the present invention toprovide a picture coding method and a picture decoding method andapparatuses and programs for executing these methods for allowingassignment of a plurality of reference indices to one reference pictureand thus improving decoding efficiency of reference indices in bothcases where a plurality of reference indices are assigned and onereference index is assigned.

In order to achieve this object, the picture coding method according tothe present invention is structured as follows. The picture codingmethod according to the present invention includes: a reference picturestorage step of storing a coded picture identified by a picture number,as a reference picture, into a storage unit; a command generation stepof generating commands indicating correspondence between referenceindices and picture numbers, said reference indices designatingreference pictures and coefficients used for generation of predictiveimages; a reference picture designation step of designating a referencepicture by a reference index, said reference picture being used whenmotion compensation is performed on a current block in a current pictureto be coded; a predictive image generation step of generating apredictive image by performing linear prediction on a block by use of acoefficient corresponding to the reference index, said block beingobtained by motion estimation within the reference picture designated inthe reference picture designation step; and a coded signal output stepof outputting a coded image signal including a coded signal obtained bycoding a prediction error, the commands, the reference index and thecoefficient, said prediction error being a difference between thecurrent block in the current picture to be coded and the predictiveimage, wherein in the coded signal output step, information indicating amaximum reference index value is coded and placed into the coded imagesignal.

Here, it may be structured so that the information indicating themaximum reference index value is placed in a picture common informationarea included in the coded image signal.

According to this structure, when picture numbers and reference indicesare corresponded to each other according to the commands in the decodingapparatus, information indicating the maximum reference index value isincluded in the coded signal. Therefore, by corresponding the picturenumbers and the reference indices according to the commands until thenumber of reference indices reaches the maximum value, all the referenceindices and the pictures numbers can be corresponded to each othereasily. As a result, it is possible not only to assign a plurality ofreference indices to one reference picture, but also to perform decodingof the reference indices efficiently in either case where a plurality ofreference indices are assigned or one reference index is assigned.

Here, it may be structured so that in the command generation step, thecommands are generated so that at least one reference picture, amongreference pictures stored in the storage unit, has a picture numberassigned a plurality of reference indices.

Also, it may be structured so that in the reference picture designationstep, when the plurality of reference indices are corresponded to thepicture number of the reference picture, one of the reference indices isselected based on coefficients corresponding respectively to saidplurality of reference indices, and in the predictive image generationstep, the linear prediction is performed using a coefficientcorresponding to the reference index selected in the reference picturedesignation step.

According to this structure, a plurality of reference indices arecorresponded to one picture. Therefore, when linear prediction isperformed using the coefficient corresponding to the reference indexdesignating the reference picture, that coefficient can be selected fromamong a plurality of coefficients. In other words, the optimumcoefficient can be selected as a coefficient used for the linearprediction. As a result, the coding efficiency can be improved.

Here, it may be structured so that in the predictive image generationstep, the linear prediction is performed using only a bit shiftoperation, an addition and a subtraction.

According to this structure, multiplications and divisions which requireheavy processing load are not used, but only bit shift operations,additions and subtractions which require smaller processing load areused, so the processing amount of linear prediction can be reduced.

Here, it may be structured so that in the predictive image generationstep, as the coefficient used in the linear prediction, only a valueindicating a direct current component in a linear prediction equation iscorresponded to the reference index.

According to this structure, values of coefficients other than the valueindicating a DC component do not need to be coded, so the codingefficiency can be improved. Also, since multiplications and divisionswhich require heavy processing load are not used but additions andsubtractions which require smaller processing load are used, theprocessing amount for linear prediction can be restrained.

Here, it may be structured so that the reference index has a firstreference index indicating a first reference picture and a secondreference index indicating a second reference picture, and in thepredictive image generation step, in the case where a method forgenerating the coefficient according to display order information ofeach of the reference pictures is used as a method for performing thelinear prediction, when a reference picture designated by the firstreference index and a reference picture designated by the secondreference index have the same display order information, the linearprediction is performed using a predetermined coefficient for each ofthe first and second reference indices instead of the coefficient.

Also, it may be structured so that the predetermined coefficients havethe same weight.

According to this structure, it is possible to determine thecoefficients for performing linear prediction even in the case where tworeference pictures have the same display order information, and thus thecoding efficiency can be improved.

Also, the picture decoding method, the picture coding apparatus, thepicture decoding apparatus, the picture coding program, the picturedecoding program, and the coded picture data of the present inventionhave the same structures, functions and effects as the picture codingmethod described above.

In addition, the picture coding method of the present invention can bestructured in any of the following manners (1) to (14).

(1) The picture coding method according to the present inventionincludes: a reference picture storage step of storing a coded pictureidentified by a picture number into a storage unit; a command generationstep of generating commands for allowing a plurality of referenceindices to refer to the same picture, said commands indicatingcorrespondence between reference indices and picture numbers, saidreference indices designating reference pictures and coefficients usedfor generation of predictive images, and said reference pictures beingreferred to when motion compensation is performed on a current block ina current picture to be coded and selected arbitrarily from among aplurality of coded pictures stored in the storage unit; a referencepicture designation step of designating a reference picture by areference index, said reference picture being referred to when motioncompensation is performed on the current block in the current picture tobe coded; a predictive image generation step of generating a predictiveimage by performing linear prediction on a block by use of a coefficientcorresponding to the reference index designating the reference picture,said block being obtained by motion estimation on the reference picturedesignated in the reference picture designation step; and a coded signaloutput step of outputting a coded image signal including a coded signalobtained by coding a prediction error, the commands, the reference indexand the coefficient, said prediction error being a difference betweenthe current block in the current frame to be coded and the predictiveimage.

(2) According to another picture coding method of the present invention,it is possible to assign a plurality of reference indices to the picturenumber of the reference picture, and it is also possible to select onereference index from among one or more reference indices correspondingto respective coded pictures in the reference picture designation stepand thus determine the coefficient used for linear prediction in thepredictive image generation step.

(3) According to another picture coding method of the present invention,at least one reference picture, among a plurality of reference picturesreferred to by one slice, has a picture number assigned a plurality ofreference indices.

(4) According to another picture coding method of the present invention,the reference index includes a first reference index indicating a firstreference picture which is arbitrarily designated from among theplurality of coded pictures and a second reference index indicating asecond reference frame which is arbitrarily designated from among theplurality of coded frames, wherein in the predictive image generationstep, the linear prediction is performed on the block by a coefficientcorresponding to the first reference index, and the linear prediction isperformed on the block by a coefficient corresponding to the secondreference index, and then a final predictive image for the block isgenerated by calculating an average of pixel values in the twopredictive images respectively obtained by the linear predictions.

(5) According to another picture coding method of the present invention,the reference index includes a first reference index indicating a firstreference picture which is arbitrarily designated from among theplurality of coded pictures and a second reference index indicating asecond reference frame which is arbitrarily designated from among theplurality of coded frames, wherein in the predictive image generationstep, the coefficient used for the linear prediction is determined bycalculating an average of the coefficients designated by the selectedfirst reference index and the selected second reference indexrespectively.

(6) According to another picture coding method of the present invention,the reference index includes a first reference index indicating a firstreference picture which is arbitrarily designated from among theplurality of coded pictures and a second reference index indicating asecond reference frame which is arbitrarily designated from among theplurality of coded frames, wherein sets of coefficients are correspondedto the first reference index and the second reference index, and in thepredictive image generation step, the predictive image is generatedusing a part of the coefficient set corresponding to one of the firstand second reference indices and a part of the coefficient setcorresponding to the other reference index.

(7) According to another picture coding method of the present invention,in an equation used for the linear prediction in the predictive imagegeneration step, neither a multiplication nor a division are used butonly a bit shift operation, an addition and a subtraction are used.Accordingly, the linear prediction can be realized by performing theprocessing with a smaller amount of calculations.

(8) According to another picture coding method of the present invention,the reference index includes a first reference index indicating a firstreference picture which is arbitrarily designated from among theplurality of coded pictures and a second reference index indicating asecond reference frame which is arbitrarily designated from among theplurality of coded frames, wherein in the predictive image generationstep, the predictive image is generated using the coefficientcorresponding to either one of the first reference index and the secondreference index which is selected, as a coefficient used for the bitshift operation, from among the coefficient sets corresponding to thefirst reference index and the second reference index, and using anaverage of the coefficients corresponding to the first reference indexand the second reference index respectively as a coefficient used forother operations.

(9) According to another picture coding method of the present invention,the reference index includes a first reference index indicating a firstreference picture which is arbitrarily designated from among theplurality of coded pictures and a second reference index indicating asecond reference frame which is arbitrarily designated from among theplurality of coded frames, wherein in the predictive image generationstep, only a value indicating a direct current component in a linearprediction equation is used as a coefficient used for the linearprediction, and one coefficient is corresponded to each of the firstreference index and the second reference index.

(10) Another picture coding method of the present invention includes: areference picture storage step of storing a coded picture identified bya picture number into a storage unit; a command generation step ofgenerating commands for allowing a plurality of reference indices torefer to the same picture, said commands indicating correspondencebetween reference indices and picture numbers, said reference indicesindicating reference pictures which are referred to when motioncompensation is performed on a current block in a current picture to becoded and selected arbitrarily from among a plurality of coded picturesstored in the storage unit; a reference picture designation step ofdesignating a reference picture by a reference index, said referencepicture being referred to when motion compensation is performed on thecurrent block in the current picture to be coded; a predictive imagegeneration step of generating a coefficient from display orderinformation of each reference picture and generating a predictive imageby performing linear prediction on a block by use of the generatedcoefficient, said block being obtained by motion estimation on thereference picture designated in the reference picture designation step;and a coded signal output step of outputting a coded image signalincluding a coded signal obtained by coding a prediction error, thecommands, the reference index and the coefficient, said prediction errorbeing a difference between the current block in the current frame to becoded and the predictive image.

(11) According to another picture coding method of the presentinvention, in the predictive image generation step, as a method forperforming the linear prediction, a method using the coefficientgenerated according to the display order information and a method usinga predetermined fixed equation are switched for use based on the displayorder information indicating temporal relationship between the referencepicture designated by the first reference index and the referencepicture designated by the second reference index.

(12) According to another picture coding method of the presentinvention, in the predictive image generation step, in the case where amethod using the coefficient generated according to display orderinformation of each of the reference pictures is used as a method forperforming the linear prediction, when a reference picture designated bythe first reference index and a reference picture designated by thesecond reference index have the same display order information, thelinear prediction is performed using a predetermined coefficient foreach of the first and second reference indices instead of thecoefficient.

(13) According to another picture coding method of the presentinvention, in the predictive image generation step, when the coefficientis generated using the display order information, the coefficient isapproximated to a power of 2 so that the linear prediction can beperformed not using a multiplication nor a division but using only a bitshift operation, an addition and a subtraction.

(14) According to another picture coding method of the presentinvention, when the approximation is performed, a round-up approximationand a round-down approximation are switched for use based on the displayorder information indicating temporal relationship between the referencepicture designated.by the first reference index and the referencepicture designated by the second reference index.

(15) A program of the present invention may be structured so as to causea computer to execute the picture coding method described in any of theabove-mentioned (1) to (14).

Furthermore, a computer readable recording medium of the presentinvention may be structured as described in the following (16) to (25).

(16) A computer readable recording medium on which a coded signal thatis a signal of a coded moving picture is recorded, wherein the codedsignal including data obtained by coding the following: a coefficientused for generation of a predictive image; commands for allowing aplurality of reference indices to refer to the same picture, saidcommands indicating correspondence between reference indices and picturenumbers, said reference indices designating reference pictures andcoefficients used for generation of predictive images, and saidreference pictures being referred to when motion compensation isperformed on a current block in a current picture to be coded andselected arbitrarily from among a plurality of coded pictures stored ina storage unit for storing the coded pictures identified by the picturenumbers; a reference index for designating the coefficient used forgeneration of the predictive image and the reference picture used whenmotion compensation is performed on the current block in the currentpicture; the predictive image which is generated by performing linearprediction on a block by use of the coefficient corresponding to thereference index designating the reference picture, said block beingobtained by motion estimation on the selected reference picture.

(17) The coded signal includes the maximum reference index value.

(18) The maximum value is placed in a picture common information areaincluded in the coded signal.

(19) A header of a slice including a plurality of blocks, a picturecommon information area, or a header of each block, which is included inthe coded signal, includes a flag indicating whether a coefficient usedfor generating a predictive image of the block using linear predictionhas been coded or not.

(20) A header of a slice including a plurality of blocks, a picturecommon information area, or a header of each block, which is included inthe coded signal, includes a flag indicating whether to generate thepredictive image of the block not using a coefficient but using apredetermined fixed equation or to generate the predictive image using apredetermined fixed equation by use of only a coefficient indicating adirect current component.

(21) A header of a slice including a plurality of blocks, a picturecommon information area, or a header of each block, which is included inthe coded signal, includes a flag indicating whether to generate thepredictive image of the block using two predetermined equations byswitching between them or to generate the predictive image using eitherone of the two equations, when the predictive image is generated using afixed equation that consists of the two equations.

(22) A header of a slice including a plurality of blocks, a picturecommon information area, or a header of each block, which is included inthe coded signal, includes a flag indicating whether or not to generatea coefficient used for generating the predictive image of the block bylinear prediction using display order information of a referencepicture.

(23) A header of a slice including a plurality of blocks, a picturecommon information area, or a header of each block, which is included inthe coded signal, includes a flag indicating whether or not toapproximate a coefficient used for generating the predictive image ofthe block by linear prediction to a power of 2.

(24) The coded signal includes a flag indicating that calculation forthe linear prediction can be carried out not using a multiplication nora division but using only a bit shift operation, an addition and asubtraction.

(25) The coded signal includes a flag indicating that calculation forthe linear prediction can be carried out using only a value indicating adirect current component.

Furthermore, the picture decoding method of the present invention can bestructured as described in the following (26) to (39).

(26) The picture decoding method of the present invention includes: acoded image information obtainment step for decoding a coded imagesignal including a coded signal obtained by coding coefficients used forgeneration of predictive images, commands for allowing a plurality ofreference indices to refer to the same picture, reference indices and aprediction error, said commands indicating correspondence between thereference indices and picture numbers, said reference indicesdesignating reference pictures and the coefficients, and said referencepictures being referred to when motion compensation is performed on acurrent block in a current picture to be coded and selected arbitrarilyfrom among a plurality of coded pictures stored in a storage unit; areference picture designation step of designating a reference pictureaccording to decoded commands and a decoded reference index, saidreference picture being used when motion compensation is performed on acurrent block in a current picture to be decoded; a predictive imagegeneration step of generating a predictive image by performing linearprediction on a block by use of a coefficient corresponding to thereference index designating the reference picture, said block beingobtained by motion estimation on the designated reference picture; and adecoded image generation step of generating a decoded image from thepredictive image and a decoded prediction error.

(27) According to another picture decoding method of the presentinvention, it is possible to assign a plurality of reference indices tothe picture number of the reference picture, and determine thecoefficient used for linear prediction in the predictive imagegeneration step using the reference index decoded in the referencepicture designation step.

(28) According to another picture decoding method of the presentinvention, at least one reference picture, among a plurality ofreference pictures referred to by one slice, has a picture numberassigned a plurality of reference indices.

(29) According to another picture decoding method of the presentinvention, the reference index includes a first reference indexindicating a first reference picture which is arbitrarily designatedfrom among a plurality of decoded pictures and a second reference indexindicating a second reference frame which is arbitrarily designated fromamong a plurality of decoded frames, wherein in the predictive imagegeneration step, the linear prediction is performed on the block by acoefficient corresponding to the first reference index, and the linearprediction is performed on the block by a coefficient corresponding tothe second reference index, and then a final predictive image for theblock is generated by calculating an average of pixel values in the twopredictive images respectively obtained by the linear prediction.

(30) According to another picture decoding method of the presentinvention, the reference index includes a first reference indexindicating a first reference picture which is arbitrarily designated bya plurality of decoded pictures and a second reference index indicatinga second reference frame which is arbitrarily designated from among aplurality of decoded frames, wherein in the predictive image generationstep, the coefficient used for the linear prediction is determined bycalculating an average of the coefficients designated by the selectedfirst reference index and the selected second reference indexrespectively.

(31) According to another picture decoding method of the presentinvention, the reference index includes a first reference indexindicating a first reference picture which is arbitrarily designatedfrom among a plurality of decoded pictures and a second reference indexindicating a second reference frame which is arbitrarily designated fromamong a plurality of decoded frames, wherein sets of coefficients arecorresponded to the first reference index and the second referenceindex, and in the predictive image generation step, the predictive imageis generated using a part of the coefficient set corresponding to one ofthe first and second reference indices and a part of the coefficient setcorresponding to the other reference index.

(32) According to another picture decoding method of the presentinvention, in an equation used for the linear prediction in thepredictive image generation step, neither a multiplication nor adivision are not used but only a bit shift operation, an addition and asubtraction are used. Accordingly, the linear prediction can be realizedby performing the processing with smaller amount of calculations.

(33) According to another picture decoding method of the presentinvention, the reference index includes a first reference indexindicating a first reference picture which is arbitrarily designatedfrom among a plurality of decoded pictures and a second reference indexindicating a second reference frame which is arbitrarily designated fromamong a plurality of decoded frames, wherein in the predictive imagegeneration step, a predictive image is generated using the coefficientcorresponding to either one of the first reference index and the secondreference index which is selected, as a coefficient used for the bitshift operation, from among the coefficient sets corresponding to thefirst reference index and the second reference index, and using anaverage of the coefficients corresponding to the first reference indexand the second reference index respectively as a coefficient used forother operations.

(34) According to another picture decoding method of the presentinvention, the reference index includes a first reference indexindicating a first reference picture which is arbitrarily designatedfrom among a plurality of decoded pictures and a second reference indexindicating a second reference frame which is arbitrarily designated fromamong a plurality of decoded frames, wherein in the predictive imagegeneration step, only a value indicating a direct current component in alinear prediction equation is used as a coefficient used for the linearprediction, and one coefficient is corresponded to each of the firstreference index and the second reference index.

(35) The picture decoding method of the present invention includes: afirst step for decoding a coded image signal including a coded signalobtained by coding commands for allowing a plurality of referenceindices to refer to the same picture, reference indices and a predictionerror, said commands indicating correspondence between the referenceindices and picture numbers, said reference indices designatingreference pictures which are referred to when motion compensation isperformed on a current block in a current picture to be coded andselected arbitrarily from among a plurality of coded pictures stored ina storage unit; a reference picture designation step of designating areference picture according to decoded commands and a decoded referenceindex, said reference picture being used when motion compensation isperformed on a current block in a current picture to be decoded; apredictive image generation step of generating a coefficient fromdisplay order information of each reference picture and generating apredictive image by performing linear prediction on a block by use ofthe generated coefficient, said block being obtained by motionestimation on the designated reference picture; and a decoded imagegeneration step of generating a decoded image from the predictive imageand a decoded prediction error.

(36) In the predictive image generation step, as a method for performingthe linear prediction, a method using a coefficient generated accordingto the display order information and a method using a predeterminedfixed equation are switched for use based on the display orderinformation indicating temporal relationship between the referencepicture designated by the first reference index and the referencepicture designated by the second reference index.

(37) According to another picture decoding method of the presentinvention, in the predictive image generation step, in the case where amethod using the coefficient generated according to display orderinformation of each of the reference pictures is used as a method forperforming the linear prediction, when a reference picture designated bythe first reference index and a reference picture designated by thesecond reference index have the same display order information, thelinear prediction is performed using a predetermined coefficient foreach of the first and second reference indices instead of thecoefficient.

(38) According to another picture decoding method of the presentinvention, in the predictive image generation step, when the coefficientis generated using the display order information, the coefficient isapproximated to a power of 2 so that the linear prediction can beperformed not using a multiplication nor a division but using a bitshift operation, an addition and a subtraction.

(39) According to another picture decoding method of the presentinvention, when the approximation is performed, a round-up approximationand a round-down approximation are switched for use based on the displayorder information indicating temporal relationship between the referencepicture designated by the first reference index and the referencepicture designated by the second reference index.

(40) A program of the present invention may be structured so as to causea computer to execute the picture decoding method described in any ofthe above-mentioned (26) to (39).

As described above, the moving picture coding method and the movingpicture decoding method of the present invention allow creation of aplurality of candidates for weighting coefficient sets used in linearprediction for generating a predictive image, and thus allow selectionof the optimum set for each block. As a result, the reference indicescan be decoded more efficiently in either case where a plurality ofreference indices are assigned or one reference index is assigned. Inaddition, since the present invention allows significant improvement ofcoding efficiency, it is very effective for coding and decoding ofmoving pictures.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a structure of a coding apparatus in afirst embodiment of the present invention.

FIG. 2 is a block diagram showing a structure of a decoding apparatus ina sixth embodiment of the present invention.

FIG. 3 is a schematic diagram for explaining a method for assigningreference indices to picture numbers.

FIG. 4 is a schematic diagram showing an example of relationship betweenreference indices and picture numbers.

FIG. 5 is a schematic diagram for explaining operations for motioncompensation.

FIG. 6 is a schematic diagram for explaining a structure of a codedstream.

FIG. 7 is a schematic diagram showing an example of weightingcoefficient sets of linear prediction coefficients.

FIG. 8 is a functional block diagram showing generation of a predictiveimage in a coding apparatus.

FIG. 9 is another functional block diagram showing generation of apredictive image in the coding apparatus.

FIGS. 10A and 10B are still other functional block diagrams showinggeneration of a predictive image in the coding apparatus.

FIG. 11 is still another functional block diagram showing generation ofa predictive image in the coding apparatus.

FIG. 12 is a schematic diagram for explaining a structure of a codedstream.

FIG. 13 is a schematic diagram showing an example of weightingcoefficient sets of linear prediction coefficients.

FIG. 14 is a schematic diagram for explaining a structure of a codedstream.

FIG. 15 is a schematic diagram showing an example of weightingcoefficient sets of linear prediction coefficients.

FIG. 16 is a functional block diagram showing generation of a predictiveimage in the coding apparatus.

FIGS. 17A and 17B are schematic diagrams for explaining a structure of acoded stream and an example of flags.

FIG. 18 is a functional block diagram showing generation of a predictiveimage in a decoding apparatus.

FIG. 19 is another functional block diagram showing generation of apredictive image in the decoding apparatus.

FIGS. 20A and 20B are still other functional block diagrams showinggeneration of a predictive image in the decoding apparatus.

FIG. 21 is still another functional block diagram showing generation ofa predictive image in the decoding apparatus.

FIG. 22 is still another functional block diagram showing generation ofa predictive image in the decoding apparatus.

FIG. 23 is a schematic diagram for explaining a structure of a codedstream.

FIG. 24 is a schematic diagram for explaining a structure of a codedstream.

FIGS. 25A, B and C are illustrations of a recording medium for storing aprogram for realizing the moving picture coding method and movingpicture decoding method in each of the above-mentioned embodiments usinga computer system.

FIG. 26 is a block diagram showing an overall configuration of a contentsupply system.

FIG. 27 is an external view of a mobile phone.

FIG. 28 is a block diagram showing a structure of the mobile phone.

FIG. 29 is a diagram showing an example of a digital broadcastingsystem.

FIG. 30 is a schematic diagram for explaining reference relationsbetween pictures in a background art.

FIGS. 31A and B are schematic diagrams for explaining reordering ofpictures in the background art.

FIG. 32 is a schematic diagram for explaining operations for motioncompensation in the background art.

FIG. 33 is a schematic diagram for explaining operations for linearprediction processing in the background art.

FIG. 34 is a schematic diagram for explaining a method for assigningreference indices to picture numbers in the background art.

FIG. 35 is a schematic diagram showing an example of relationshipbetween reference indices and picture numbers in the background art.

FIG. 36 is a schematic diagram for explaining a structure of a codedstream in the background art.

FIG. 37 is a schematic diagram showing an example of weightingcoefficient sets of linear prediction coefficients in the backgroundart.

FIG. 38 is a schematic diagram for explaining a structure of a codedstream in the background art.

FIG. 39 is a schematic diagram for explaining relationship betweenpicture numbers and display order information.

FIGS. 40A and 40B are schematic diagrams for explaining a structure of acoded stream and an example of flags.

FIGS. 41A and 41B are schematic diagrams for explaining a structure of acoded stream and an example of flags.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 is a block diagram showing a structure of a moving picture codingapparatus in the first embodiment of the present invention. The movingpicture coding method executed in this moving picture coding apparatus,specifically, (1) an overview of coding, (2) a method for assigningreference indices and (3) a method for generating a predictive image,will be explained in this order using the block diagram as shown in FIG.1.

(1) Overview of Coding

A moving picture to be coded is inputted to a picture memory 101 on apicture-by-picture basis in display order, and the inputted pictures arereordered into coding order. FIGS. 31A and 31B are diagrams showing anexample of reordering of pictures. FIG. 31A shows an example of picturesin display order, and FIG. 31B shows an example of the picturesreordered into coding order. Here, since pictures B3 and B6 refer bothtemporally preceding and subsequent pictures, the reference picturesneed to be coded before coding these current pictures and thus thepictures are reordered in FIG. 31B so that pictures P4 and P7 are codedearlier. Each of the pictures is divided into blocks, each of which iscalled a macroblock of horizontal 16 by vertical 16 pixels, for example,and the following proceeding is performed on a block-by-block basis.

An input image signal read out from the picture memory 101 is inputtedto a difference calculation unit 112. The difference calculation unit112 calculates a difference between the input image signal and thepredictive image signal outputted from a motion compensation coding unit107, and outputs the obtained difference image signal (prediction errorsignal) to a prediction error coding unit 102. The prediction errorcoding unit 102 performs image coding processing such as frequencytransformation and quantization, and outputs a coded error signal. Thecoded error signal is inputted to a prediction error decoding unit 104,which performs image decoding processing such as inverse-quantizationand inverse-frequency transformation, and outputs a decoded errorsignal. An addition unit 113 adds the decoded error signal and thepredictive image signal to generate a reconstructed image signal, andstores, into a picture memory 105, the reconstructed image signals whichcould be referred in the following inter-picture prediction out of theobtained reconstructed image signals.

On the other hand, the input image signal read out per macroblock fromthe picture memory 101 is also inputted into a motion vector estimationunit 106. Here, the reconstructed image signals stored in the picturememory 105 are searched so as to estimate an image area which is themost alike to the input image signal and thus determine a motion vectorpointing to the position of the image area. The motion vector estimationis performed per block that is a subdivision of a macroblock, and theobtained motion vectors are stored in a motion vector storage unit 108.

At this time, since a plurality of pictures can be used for reference inH.26L which is now under consideration for standardization,identification numbers for designating reference pictures are needed perblock. The identification numbers are referred to as reference indices,and a reference index/picture number conversion unit 111 establishescorrespondence between the reference indices and the picture numbers ofthe pictures stored in the picture memory 105 so as to allow designationof the reference pictures. The operation in the reference index/picturenumber conversion unit 111 will be explained in detail in the section(2).

The motion compensation coding unit 107 extracts the image area that ismost suitable for the predictive image from among the reconstructedimage signals stored in the picture memory 105, using the motion vectorsestimated by the above-mentioned processing and the reference indices.Pixel value conversion processing such as interpolation processing bylinear prediction is performed on the pixel values of the obtained imagearea so as to obtain the final predictive image. The linear predictioncoefficients used for that purpose are generated by the linearprediction coefficient generation unit 110, and stored in the linearprediction coefficient storage unit 109. This predictive imagegeneration method will be explained in detail in the section (3).

The coded stream generation unit 103 performs variable length coding forthe coded information such as the linear prediction coefficients, thereference indices, the motion vectors and the coded error signalsoutputted as a result of the above series of processing so as to obtaina coded stream to be outputted from this coding apparatus.

The flow of operations in the case of inter-picture prediction codinghas been described above, and a switch 114 and a switch 115 switchbetween inter-picture prediction coding and intra-picture predictioncoding. In the case of intra-picture prediction coding, a predictiveimage is not generated by motion compensation, but a difference imagesignal is generated by calculating a difference between a current areaand a predictive image of the current area which is generated from acoded area in the same picture. The prediction error coding unit 102converts the difference image signal into the coded error signal in thesame manner as inter-picture prediction coding, and the coded streamgeneration unit 103 performs variable length coding for the signal toobtain a coded stream to be outputted.

(2) Method for Assigning Reference Indices

Next, the method by which the reference index/picture number conversionunit 111 as shown in FIG. 1 assigns reference indices will be explainedusing FIG. 3 and FIG. 4.

FIG. 3 is a diagram for explaining the method for assigning tworeference indices to picture numbers. Supposing that there is a sequenceof pictures ordered in display order as shown in this diagram, picturenumbers are assigned to the pictures in coding order. Commands forassigning reference indices to picture numbers are described in a headerof each slice that is a subdivision of a picture, as the unit of coding,and thus the assignment thereof is updated every time one slice iscoded. The command indicates, in series by the number of referenceindices, the differential value between a picture number which isassigned a reference index currently and a picture number which isassigned a reference index immediately before the current assignment.

Taking the first reference indices in FIG. 3 as an example, since “−1”is given first as a command, a picture with its picture number 15 isassigned to the reference index number 0 by subtracting 1 to the currentpicture number 16. Next, “−4” is given as a command, a picture with itspicture number 11 is assigned to the reference index number 1 bysubtracting 4 from the picture number 15 which has been assigned justbefore it. Each of the following picture numbers is assigned in the samemanner. The same applies to the second reference indices.

According to the conventional reference index assignment method as shownin FIG. 34, all the reference indices are corresponded to respectivepicture numbers. On the other hand, in the example of FIG. 3, althoughexactly the same assignment method is used, a plurality of referenceindices are corresponded to the same picture number by changing thevalues of the commands.

FIG. 4 shows the result of assignment of the reference indices. Thisdiagram shows that the first reference index and the second referenceindex are assigned to each picture number separately, but a plurality ofreference indices are assigned to one picture number in some cases. Inthe coding method of the present invention, it is assumed that aplurality of reference indices are assigned to at least one picturenumber, like this example.

If the reference indices are used only for determination of referencepictures, the conventional method of one-to-one assignment of referenceindices to picture numbers is the most efficient coding method. However,in the case where a weighting coefficient set of linear predictioncoefficients is selected for generation of a predictive image usingreference indices, the same linear prediction coefficients have to beused for all the blocks having the same reference pictures, so there isan extremely high possibility that the optimum predictive image cannotbe generated.

So, if it is possible to assign a plurality of reference indices to onepicture number as in the case of the present invention, the optimumweighting coefficient set of linear prediction coefficients can beselected for each block from among a plurality of candidate sets even ifall the blocks have the same reference picture, and thus the predictiveimage with its coding efficiency being higher can be generated.

Note that, the above description shows the case where the picturenumbers are given assuming that all the reference pictures are stored ina reference memory. However, a current picture is given a picture numberwhich is larger by one than the number of a picture which has been codedimmediately before the current picture, only when the current picturewhich has been coded lastly is stored, so the continuity of the picturenumbers are maintained in the reference memory even if some pictures arenot stored, and thus the above-mentioned method can be used withoutchange.

(3) Method for Generating Predictive Image

Next, the predictive image generation method in the motion compensationcoding unit 107 as shown in FIG. 1 will be explained using FIG. 5.Although the predictive image generation method by linear prediction isexactly the same as the conventional method, flexibility in selection oflinear prediction coefficients is increased because a plurality ofreference index numbers can be corresponded to the same picture.

The picture B16 is a current B-picture to be coded, and the blocks BL01and BL02 are current blocks to be coded which belong to the B-picture.The picture P11 and the picture B15 are used as the first referencepicture and the second reference picture for BL01, and the predictiveimage is generated with reference to the blocks BL11 and BL21 belongingto the pictures P11 and B15 respectively. In the same manner, thepicture P11 and the picture B15 are used as the first reference pictureand the second reference picture for BL02, the predictive image isgenerated with reference to the blocks BL12 and BL22 belonging to thosereference pictures respectively.

Although both BL01 and BL02 refer to the same pictures as their firstreference picture and the second reference picture, it is possible toassign different values to the first reference index ref1 and the secondreference index ref2 for BL01 and BL02 by using the reference indexassignment method as explained in the above (2). Taking FIG. 4 as anexample, 1 and 3 are assigned to the first reference index correspondingto the picture number 11, whereas 1 and 6 are assigned to the secondreference index corresponding to the picture number 15.

As a result, four combinations of these reference indices, (ref1,ref2)=(1, 1), (1, 6), (3, 1) and (3, 6) are supposed, and thus it ispossible to select the combination for deriving the optimum weightingcoefficient set on a per-block basis from among these combinations. InFIG. 5, ref1=1 and ref2=1 are assigned for BL01, and ref1=3 and ref2=6are assigned for BL02, for example.

According to the conventional reference assignment method as shown inFIG. 35, only one combination of (ref1, ref2)=(1, 1) can be selected forboth BL01 and BL02 in the case of FIG. 5, and thus only one weightingcoefficient set of linear prediction coefficients can be selected, too.On the other hand, according to the present invention, four options areavailable, and it can be said that the possibility for selection of theoptimum weighting coefficient set is increased.

A coded stream of one picture is made up of a picture common informationarea and a plurality of slice data areas. FIG. 6 shows the structure ofthe slice data area in the coded stream. The slice data area is furthermade up of a slice header area and a plurality of block data areas. Thisdiagram shows each of the block areas corresponding to BL01 and BL02 inFIG. 5 as an example of the block data area. “ref1” and “ref2” includedin BL01 designate the first reference index and the second referenceindex respectively indicating two pictures referred to by the blockBL01.

Also, in the slice header area, data (pset0, pset1, pset2, . . . ) forgiving the weighting coefficient sets for performing the above-mentionedlinear prediction is described for ref1 and ref2 respectively. In thisarea, “pset”s of the number equivalent to the number of the referenceindices as explained in (2) can be set. To be more specific, in the casewhere ten reference indices from 0 to 9 are used as each of the firstreference index and the second reference index, ten “pset” s from 0 to 9can also be set for each of ref1 and ref2.

FIG. 7 shows an example of tables of the weighting coefficient setsincluded in the slice header area. Each data indicated by an identifierpset has four values w1, w2, c and d, and these tables are structured sothat the data can be directly referred to by the values of ref1 andref2. Also, command sequences idx_cmd1 and idx_cmd2 for assigning thereference indices to the picture numbers are described in the sliceheader area.

Using ref1 and ref2 described in BL01 in FIG. 6, one set of weightingcoefficients is selected from each of the tables for ref1 and ref2 inFIG. 7. By performing linear prediction on the pixel values of thereference images using these two sets of weighting coefficients, apredictive image is generated.

As described above, using the coding method in which a plurality ofreference indices are assigned to one picture number, a plurality ofcandidates for weighting coefficient sets of linear predictioncoefficients can be created, and thus the best one can be selected fromamong them. For example, in the case where two first reference indicesand two second reference indices are assigned, four sets of weightingcoefficients are available as candidates for selection, and in the casewhere three first reference indices and three second reference indicesare assigned, nine sets of weighting coefficients are available ascandidates for selection.

This linear prediction method has a dramatic effect particularly in thecase where the brightness of the entire picture or a part thereofchanges significantly like fade and flash. In many cases, the degree ofchange in brightness differs from part to part of a picture. Therefore,the structure of the present invention in which the best one can beselected for each block from among a plurality of weighting coefficientsets is very effective in image coding.

Here, the flow of processing from determination of weighting coefficientsets up to generation of a predictive image will be explained in detail.

FIG. 8 is a functional block diagram showing the functional structurefor generating a predictive image in the linear prediction coefficientgeneration unit 110, the linear prediction coefficient storage unit 109and the motion compensation coding unit 107.

A predictive image is generated via the linear prediction coefficientgeneration unit 110, the linear prediction coefficient storage unit 109a, the linear prediction coefficient storage unit 109 a, the averagecalculation unit 107 a and the linear prediction operation unit 107 b.

The weighting coefficient sets generated by the linear predictioncoefficient generation unit 110 are stored in the linear predictioncoefficient storage unit 109 a and the linear coefficient storage unit109 b. The average calculation unit 107 a obtains, from the linearprediction coefficient storage unit 109 a, one set of weightingcoefficients (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected by the firstreference index ref1 determined by the motion estimation processing, andfurther obtains, from the linear coefficient storage unit 109 b, one setof weighting coefficients (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected bythe second reference index ref2.

Then, the average calculation unit 107 a calculates the average, forrespective parameters, of the weighting coefficient sets obtained fromthe linear prediction coefficient storage units 109 a and 109 b so as toconsider it as the weighting coefficient set (w1, w2, c, d) to be usedfor the actual linear prediction, and outputs it to the linearprediction operation unit 107 b. The linear prediction operation unit107 b calculates the predictive image using the equation 1 based on theobtained weighting coefficient set (w1, w2, c, d) for output.

FIG. 9 is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient generation unit 110, thelinear prediction coefficient storage unit 109 a, the linear predictioncoefficient storage unit 109 b, the linear prediction operation unit 107c, the linear prediction operation unit 107 d and the averagecalculation unit 107 e.

The weighting coefficient sets generated by the linear predictioncoefficient generation unit 110 are stored in the linear predictioncoefficient storage unit 109 a and the linear prediction coefficientstorage unit 109 b. The linear prediction operation unit 107 c obtains,from the linear prediction coefficient storage unit 109 a, one set ofweighting coefficients (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected bythe first reference index refl determined by the motion estimationprocessing, and calculates the predictive image using the equation 1based on the weighting coefficient set for output to the averagecalculation unit 107 e.

In the same manner, the linear prediction operation unit 107 d obtains,from the linear prediction coefficient storage unit 109 b, one set ofweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected bythe second reference index determined by the motion estimationprocessing, and calculates the predictive image using the equation 1based on the weighting coefficient set for output to the averagecalculation unit 107 e.

The average calculation unit 107 e calculates the average of respectivepixel values of predictive images which are outputted from the linearprediction operation unit 107 c and the linear prediction operation unit107 d respectively so as to generate the final predictive image foroutput.

FIG. 10A is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient generation unit 110, thelinear prediction coefficient storage unit 109 c, the linear predictionstorage unit 109 d, the average calculation unit 107 f, and the linearprediction operation unit 107 g.

The weighting coefficient sets generated by the linear predictioncoefficient generation unit 110 are stored in the linear predictioncoefficient storage unit 109 c and the linear prediction coefficientstorage unit 109 d. The average calculation unit 107 f obtains, from thelinear prediction coefficient storage unit 109 c, the parameters ofc_(—)1 and d_(—)1 in one set of weighting coefficients (w1_(—)1,w2_(—)1, c_(—)1, d_(—)1) selected by the first reference index refldetermined by the motion estimation processing, and similarly, obtains,from the linear prediction coefficient storage unit 109 d, theparameters of c_(—)2 and d_(—)2 in one set of weighting coefficients(w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected by the second referenceindex ref2. The average calculation unit 107 f calculates the average ofc_(—)1 and c_(—)2 and the average of d_(—)1 and d_(—)2 obtained from thelinear prediction coefficient storage unit 109 c and the linearprediction coefficient storage unit 109 d so as to obtain c and d foroutput to the linear prediction operation unit 107 g.

Also, the linear prediction operation unit 107 g obtains the parameterof w1_(—)1 in the above-mentioned weighting coefficient set (w1_(—)1,w2_(—)1, c_(—)1, d_(—)1) from the linear prediction coefficient storageunit 109 c, obtains the parameter of w2_(—)2 in the above-mentionedweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) from thelinear prediction coefficient storage unit 109 d, and obtains c and dwhich are the averages calculated by the average calculation unit 107 f,and then calculates the predictive image using the equation 1 foroutput.

To be more specific, when determining the weighting coefficient set (w1,w2, c, d) which is actually used for linear prediction from among theweighting coefficient set (w1_(—)2, w2_(—)1, c_(—)1, d_(—)1) obtainedfrom the linear prediction coefficient storage unit 109 c and theweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) obtainedfrom the linear prediction coefficient storage unit 109 d, the linearprediction operation unit 107 g uses the following rule.

-   -   w1=w1_(—)1    -   w2=w2_(—)2    -   c=(average of c_(—)1 and c_(—)2)    -   d=(average of d_(—)1 and d_(—)2)

As described above, in generation of the predictive image as explainedin FIG. 10A, the linear prediction coefficient storage unit 109 c doesnot need w2_(—)1 in the weighting coefficient set. Therefore, w2 is notrequired for the weighting coefficient set for ref1, and thus dataamount of a coded stream can be reduced.

Also, the linear prediction coefficient storage unit 109 d does not needw1_(—)2 in the weighting coefficient set. Therefore, w1 is not requiredfor the weighting coefficient set for ref2, and thus data amount of acoded stream can be reduced.

FIG. 10B is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient generation unit 110, thelinear prediction coefficient storage unit 109 e, the linear predictioncoefficient storage unit 109 f and the linear prediction operation unit107 h.

The weighting coefficient sets generated by the linear predictioncoefficient generation unit 110 are stored in the linear predictioncoefficient storage unit 109 e and the linear prediction coefficientstorage unit 109 f. The linear prediction operation unit 107 h obtains,from the linear prediction coefficient storage unit 109 e, theparameters of w1_(—)1, c_(—)1 and d_(—)1 which are a part of one set ofweighting coefficients (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected bythe first reference index refl determined by the motion estimationprocessing, and similarly, obtains, from the linear predictioncoefficient storage unit 109 f, the parameter of w2_(—)2 which is a partof one set of weighting coefficients (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2)selected based on the second reference index ref2. The linear predictionoperation unit 107 h calculates a predictive image using the equation 1based on w1_(—)1, c_(—)1, d_(—)1, w2_(—)2 obtained from the linearprediction coefficient storage unit 109 e and the linear predictioncoefficient storage unit 109 f for output.

To be more specific, when determining the weighting coefficient set (w1,w2, c, d) which is actually used for linear prediction from among theweighting coefficient set (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) obtainedfrom the linear prediction coefficient storage unit 109 e and theweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) obtainedfrom the linear prediction coefficient storage unit 109 f, the linearprediction operation unit 107 h uses the following rule.

-   -   w1=w1_(—)1    -   w2=w2_(—)2    -   c=c_(—)1    -   d=d_(—)1

In generation of the predictive image as explained in FIG. 10B, thelinear prediction coefficient storage unit 109 e does not need w2_(—)1in the weighting coefficient set. Therefore, w2 is not required for theweighting coefficient set for ref1, and thus data amount of a codedstream can be reduced.

Also, the linear prediction coefficient storage unit 109 f does not needw1_(—)2, c_(—)2 and d_(—)2 in the weighting coefficient set. Therefore,w1, c and d are not required for the weighting coefficient set for ref2,and thus data amount of a coded stream can be reduced.

Furthermore, it is also possible to use one or more parameters out ofw1, w2, c and d as fixed values. FIG. 11 is a functional block diagramin the case where only “d” as a fixed value is used for the functionalstructure in FIG. 10A. A predictive image is generated via the linearprediction coefficient generation unit 110, the linear predictioncoefficient storage unit 109 i, the linear prediction coefficientstorage unit 109 j, the average calculation unit 107 j and the linearprediction operation unit 107 k.

The coefficients selected by the first reference index refl from thelinear prediction coefficient storage unit 109 i are only (w1_(—)1,c_(—)1), and the coefficients selected by the second reference index ref2 from the linear prediction coefficient storage unit 109 j are only(w2_(—)2, c_(—)2). The average calculation unit 107 j calculates theaverage of c_(—)1 and c_(—)2 obtained from the linear predictioncoefficient storage unit 109 i and the linear prediction coefficientstorage unit 109 j so as to obtain “c” and outputs it to the linearprediction operation unit 107 k.

The linear prediction operation unit 107 k obtains the parameter ofw1_(—)1 from the linear prediction coefficient storage unit 109 i, theparameter of w2_(—)2 from the linear prediction coefficient storage unit109 j, and the parameter of c from the average calculation unit 107 j,and then calculates the predictive image based on the equation 1 using apredetermined fixed value as a parameter of d, and outputs thepredictive image. To be more specific, the following values are enteredas the coefficients (w1, w2, c, d) of the equation 1.

-   -   w1=w1_(—)1    -   w2=w2_(—)2    -   c=(average of c_(—)1 and c_(—)2)    -   d=(fixed value)

Assignment of the above values into the equation 1 provides thefollowing equation 1a.P(i)=(w1_(—)1×Q1(i)+w2_(—)2×Q2(i))/pow(2, d)+(c _(—)1+c_(—)2)/2  Equation 1a(where pow(2, d) indicates the “d” th power of 2)

Furthermore, by modifying the equation 1a, the following equation 1b isobtained. It is possible for the linear prediction operation unit 107 kto handle the linear prediction operation method in exactly the samemanner in either format of the equation 1b or the equation 1.P(i)=(w1_(—)1×Q1(i)/pow(2, d−1)+c _(—)1+w2_(—)2×Q2(i)/pow(2, d−1)+c_(—)2)/2  Equation 1b(where pow(2, d−1) indicates the “d−1”th power of 2)

Although pow(2, d−1) is used in the equation 1b, the system may bestructured using pow(2, d′) by entering d′ (assuming d′=d−1) into thelinear prediction operation unit 107 k because d is a fixed value.

In addition, in generation of the predictive image as explained in FIG.11, the linear prediction coefficient storage unit 109 i needs onlyw1_(—)1 and c_(—)1 among the weighting coefficient sets for it, and thelinear prediction coefficient storage unit 109 j needs only w2_(—)2 andc_(—)2 among the weighting coefficient sets for it. Therefore, there isno need to code the parameters other than the above required parameters,and thus the data amount of the coded stream can be reduced.

Note that it is possible to use one predetermined fixed value as a valueof d in any cases, but the fixed value can be switched on a per-slicebasis by describing the fixed value in the slice header. Similarly, thefixed value can be switched on a per-picture basis or a per-sequencebasis by describing it in the picture common information area or thesequence common information area.

FIG. 12 shows an example of a structure of a slice data area in the casewhere the above-mentioned linear prediction method is used. This isdifferent from FIG. 6 in that only “d” is described in the slice headerarea and only w1_(—)1 and c_(—)1 are described as pset for refl. FIG. 13shows tables showing an example of the above weighting coefficient setsincluded in the slice header area. Each data indicated by the identifier“pset” has two values of w1_(—)1 and c_(—)1 or w2_(—)2 and c_(—)2, andstructured so as to be directly referred to by the values of ref1 andref2.

Note that the linear prediction coefficient generation unit 110generates the weighting coefficient sets by examining thecharacteristics of a picture, and the motion compensation coding unit107 creates a predictive image using any of the methods as explained inFIG. 8, FIG. 9, FIG. 10 and FIG. 11 and determines the combination oftwo reference indices ref1 and ref2 so as to minimize the predictionerror. In the case where any of the methods of FIG. 10A, FIG. 10B andFIG. 11 which do not require all the parameters is used, it is possibleto omit the processing of creating unnecessary parameters in the stagewhere the linear prediction coefficient generation unit 110 in thecoding apparatus creates the weighting coefficient sets.

In the methods of FIG. 10A, FIG. 10B and FIG. 11, it is possible for thelinear prediction coefficient generation unit 110 to search for andcreate the optimum weighting coefficient sets for ref1 and ref2, w1_(—)1and w2_(—)2, for example, separately, when creating such weightingcoefficient sets. In other words, using these methods, it is possible toreduce the amount of processing executed by the coding apparatus forcreating weighting coefficient sets.

Note that the above-mentioned coding methods are concerned with aB-picture having two reference pictures, but it is also possible toexecute the same processing in the single picture reference coding modefor a P-picture or a B-picture having only one reference picture. Inthis case, using only one of the first reference index and the secondreference index, pset and idx_cmd for either ref1 or ref2 are onlydescribed in the slice header area included in the coded stream of FIG.6, according to the reference index described in the block data area.

In addition, as a linear prediction method, the following equation 3 isused instead of the equation 1 as explained in the conventional method.In this case, Q1(i) is a pixel value of a block which is referred to,P(i) is a pixel value of a predictive image of a current block to becoded, and w1, w2, c and d are linear prediction coefficients given bythe selected weighting coefficient set.P(i)=(w1×Q1(i)+w2×Q1(i))/pow(2, d)+c  Equation 3

-   -   (where pow(2, d) indicates the “d” th power of 2)

Note that it is possible to use the equation 4 as a linear predictionequation, instead of the equation 3. In this case, Q1(i) is a pixelvalue of a block which is referred to, P(i) is a pixel value of apredictive image of a current block to be coded, and w1, c and d arelinear prediction coefficients given by the selected weightingcoefficient set.P(i)=(w1×Q1(i))/pow(2, d)+c  Equation 4

-   -   (where pow(2, d) indicates the “d” th power of 2)

Use of the equation 1 and the equation 3 requires four parameters w1,w2, c and d, whereas use of the equation 4 requires only threeparameters w1, c and d for linear prediction. In other words, in thecase where either one of the first reference index and the secondreference index is used for a picture as a whole, like a P-picture, itis possible to reduce the number of data items of each weightingcoefficient set to be described in the slice header area to three.

When the equation 3 is used, it is possible to realize linear predictionwhich is available for both B-pictures and P-pictures adaptively withoutchange in structure. When the equation 4 is used, the data amount to bedescribed in the header area of a P-picture can be reduced, and thus itis possible to achieve reduction of the processing amount because ofsimplified calculation. However, since the reference index assignmentmethod suggested by the present invention can directly be applied to anyof the above methods, a predictive image with its high coding efficiencycan be created, which is extremely effective in image coding.

By the way, the pictures which are referred to in motion compensationare determined by designating the reference indices assigned torespective pictures. In that case, the maximum number of pictures whichare available for reference has been described in the picture commoninformation area in the coded stream.

FIG. 38 is a schematic diagram of a coded stream in which the maximumnumber of pictures which are available for reference is described. Asthis diagram shows, the maximum number of pictures for ref1 Max_pic1 andthe maximum number of pictures for ref2 Max_pic2 are described in thepicture common information in the coded stream.

Information required for coding is not the maximum number of realpictures themselves, but the maximum reference index value available fordesignating pictures.

Since one reference index is assigned to one picture in the conventionalmethod, the above-mentioned description of the maximum number ofpictures causes no contradiction. But the difference in number betweenreference indices and pictures has a significant influence on the casewhere a plurality of reference indices are assigned to one picturenumber, like the present invention.

As described above, command sequences idx_cmd1 and idx_cmd2 aredescribed in a coded stream in order to assign reference indices topicture numbers. The picture numbers and the reference indices arecorresponded to each other based on each command in these commandsequences idx_cmd1 and idx_cmd2. For that purpose, knowing the maximumreference index value shows that all the reference indices and picturenumbers have been corresponded to each other, namely, the end of thecommands in the command sequences idx_cmd1 and idx_cmd2.

Therefore, in the present embodiment, the maximum number of availablereference indices, instead of the maximum number of pictures in thebackground art, is described in the picture common information areabeing the header of the picture. Or, both of the maximum number ofpictures and the maximum number of reference indices are described.

FIG. 23 shows a picture common information area in a coded stream of apicture in which the maximum numbers of reference indices are described.In the picture common information area, the maximum number of referenceindices available for ref1 Max_idx1 and the maximum number of referenceindices available for ref2 Max_idx2 are described.

In FIG. 23, the maximum number of reference indices is described in thepicture common information, but it may be structured so that the maximumnumber of reference indices is described in the slice data area as wellas in the picture common information. For example, the maximum number ofreference indices required for each slice can be clearly described, inthe case where the maximum number of required reference indices for eachslice is significantly different from the maximum number thereofdescribed in the picture common information area, from slice to slice,for example, the maximum number of reference indices in a picture is 8,the maximum number of reference indices required for the slice 1 in thepicture is 8, and the maximum number of reference indices required forthe slice 2 is 4.

In other words, it may be structured so that the maximum number ofreference indices described in the picture common information is set asa default value common to all the slices in the picture and the maximumnumber of reference indices required for a slice, which is differentfrom the default value, is described in the slice header.

Although FIG. 23 and FIG. 38 show the examples where a coded stream ismade up of a picture common information area and slice data areas, thepicture common information area and the slice data areas can be handledas separate coded streams in exactly the same manner as one codedstream.

Second Embodiment

The moving picture coding method in the second embodiment of the presentinvention will be explained. Since the structure of the codingapparatus, the flow of coding processing and the reference indexassignment method are exactly the same as those in the first embodiment,the explanation thereof is not repeated here.

In the first embodiment, linear prediction is performed on each pixelfor generating a predictive image in motion compensation using theequation 1, the equation 3 or the equation 4. However, since all ofthese equations include multiplications, which cause significantincrease of processing amount in considering that these multiplicationsare performed on all the pixels.

So, it is possible to use the equation 5 instead of the equation 1, theequation 6 instead of the equation 3, and the equation 7 instead of theequation 4. These equations allow calculations only using bit shiftoperations without using multiplications, and thus allows reduction ofprocessing amount. In the following equations, Q1(i) and Q2(i) are pixelvalues of referred blocks, P(i) is a pixel value of a predictive imageof a current block to be coded, and m, n and c are linear predictioncoefficients given by a selected weighting coefficient set.P(i)=±pow(2, m)×Q1(i)±pow(2, n)×Q2(i)+c  Equation 5P(i)=±pow(2, m)×Q1(i)±pow(2, n)×Q1(i)+c  Equation 6P(i)=±pow(2, m)×Q1(i)+c  Equation 7

-   -   (where pow(2, m) indicates “m”th power of 2, and pow(2, n)        indicates “n”th power of 2)

As is the case. with the first embodiment, the equation 5 is used forgenerating a predictive image with reference to two pictures at a time,and the equation 6 or the equation 7 is used for generating a predictiveimage with reference to only one picture. Since these equations requireidentifiers indicating plus and minus signs, weighting coefficient setsrequired for prediction operations are (sign1, m, sign2, n, c) for theequations 5 and 6, and (sign1, m, c) for the equation 7. “sing1” and“sign2” are parameters identifying the first and second plus and minussigns, respectively. Although the number of parameters is larger thanthat in the first embodiment, there is little increase in processingamount because both sign1 and sign2 can be represented by 1 bit.

Here, the flow of processing from determination of weighting coefficientsets until generation of a predictive image with reference to twopictures at a time using the equation 5 will be explained in detail.

First, the case where the functional structure for generation of apredictive image is that as shown in FIG. 8 will be explained. Theaverage calculation unit 107 a obtains the weighting coefficient set(sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1, c_(—)1) from the linearprediction coefficient storage unit 109 a. Also, the average calculationunit 107 a obtains the weighting coefficient set (sign1_(—)2, m_(—)2,sign2_(—)2, n_(—)2, c_(—)2) from the linear prediction coefficientstorage unit 109 b.

The average calculation unit 107 a calculates, for respectiveparameters, the average of the weighting coefficient sets obtained fromthe linear prediction coefficient storage unit 109 a and the linearprediction coefficient storage unit 109 b so as to consider the averageas the weighting coefficient set (sign1, m, sign2, n, c). The linearprediction operation unit 107 b calculates the predictive image usingthe equation 5 based on the weighting coefficient set (sign1, m, sign2,n, c) outputted from the average calculation unit 107 a.

Note that FIG. 8 shows the weighting coefficient set (w1_(—)1, w2_(—)1,c_(—)1, d_(—)1) and the like obtained from the linear predictioncoefficient storage unit 109 a and the like, which are calculated in thecase where the equation 1 is used as explained in the first embodiment,and does not show the parameters used in the case where the predictiveimage is obtained using the equation 5, but the parameters used in theformer case can be replaced by the parameters in the latter case as theyare. The same goes for the cases of FIG. 9 and FIG. 10 to be describedlater.

Next, the case where the functional structure for generation of apredictive image is that as shown in FIG. 9 will be explained. Thelinear prediction operation unit 107 c calculates a predictive image 1based on the weighting coefficient set (sign1_(—)1, m_(—)1, sign2_(—)1,n_(—)1, c_(—)1) obtained from the linear prediction coefficient storageunit 109 a. The linear prediction operation unit 107 d calculates apredictive image 2 based on the weighting coefficient set (sign1_(—)2,m_(—)2, sign2_(—)2, n_(—)2, c_(—)2) obtained from the linear predictioncoefficient storage unit 109 b. And the average calculation unit 107 ecalculates, for respective pixels, the average of the predictive imagescalculated by the linear prediction operation units 107 c and 107 d soas to obtain a predictive image.

In this case, the linear prediction operation unit 107 c firstcalculates the predictive image using the equation 5 based on theweighting coefficient set (sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1,c_(—)1), so it is possible to calculate the predictive image using bitshift operations without using multiplications. The same goes for thelinear prediction operation unit 107 d. On the other hand, in the caseof FIG. 8, since the average of the weighting coefficient set(sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1, c_(—)1) and the weightingcoefficient set (sign1_(—)2, m_(—)2, sign2_(—)2, n_(—)2, c_(—)2) iscalculated first, the average of m_(—)1and m_(—)2or the average ofn_(—)1 and n_(—)2, namely, the exponents of 2, could not be integers,and thus there is a possibility that the processing amount increases.Also, if the exponents of 2 are rounded off to integers, there isanother possibility that the error increases.

Next, the case where a predictive image is generated in the functionalstructure as shown in FIG. 10A will be explained. The linear predictionoperation unit 107 g calculates a predictive image using the equation 5,based on the parameters sign1_(—)1 and m_(—)1which are obtained from thelinear prediction coefficient storage unit 109 c and used for bit shiftoperations; the parameters sign2_(—)2 and n_(—)2 which are obtained fromthe linear prediction coefficient storage unit 109 d and used for bitshift operations; and the average c calculated by the averagecalculation unit 107 f, of the parameters c_(—)1 and c_(—)2 which areobtained from the linear prediction coefficient storage units 109 c and109 d.

In this case, since the coefficients used for bit shift operations arethe values which are obtained directly from the linear predictioncoefficient storage unit 109 c or the linear prediction coefficientstorage unit 109 d, the exponents of 2 in the equation 5 are integers.Therefore, the calculations can be performed using bit shift operations,and thus the processing amount can be reduced.

The case where a predictive image is generated in the functionalstructure as shown in FIG. 10B will be explained. The linear predictionoperation unit 107 h calculates a predictive image using the equation 5based on the parameters sign1_(—)1, m_(—)1 and c_(—)1 which are obtainedfrom the linear prediction coefficient storage unit 109 e and theparameters sign2_(—)2 and n_(—)2 which are obtained from the linearprediction coefficient storage unit 109 f.

In this case, since the coefficients used for bit shift operations arevalues which are obtained directly from the linear predictioncoefficient storage unit 109 e or the linear prediction coefficientstorage unit 109 f, the exponents of 2 in the equation 5 are integers.Therefore, the predictive image can be calculated using bit shiftoperations, and thus the processing amount can be reduced.

In the cases of FIGS. 10A and 10B, there are parameters which do notneed to be added to the coded stream for transmission, as is the casewith the cases of FIGS. 10A and 10B in the first embodiment, and thedata amount of the coded stream can be reduced.

Using the linear prediction equations as explained in the secondembodiment, the calculations can be performed using bit shift operationswithout using multiplications, so the processing amount can besignificantly reduced from that in the first embodiment.

In the present embodiment, linear prediction is performed using theequations 5, 6 and 7 instead of the equations 1, 3 and 4, and using theparameter set to be coded (sign1, m, sign2, n, c) instead of (w1, w2, c,d), so the calculations can be realized only using bit shift operationsand thus reduction of processing amount is achieved. However, it is alsopossible, as another approach, to use the equations 1, 3 and 4 and (w1,w2, c, d) as they are, by limiting the selectable values of w1 and w2 toonly the values available for bit shift operations, so the calculationscan be realized only using bit shift operations and thus reduction ofprocessing amount is achieved.

When determining the values of w1 and w2, the linear predictioncoefficient generation unit 110 in FIG. 1 selects, as one of thechoices, only the values available for bit shift operations anddescribes the selected values directly into the coded streams in FIG. 6and FIG. 12, as the values of w1 and w2 therein. As a result, it ispossible to reduce the processing amount for linear prediction even inexactly the same structure as that of the first embodiment. It is alsopossible to determine the coefficients easily because the choices of thecoefficients are limited.

In addition, as a method for such limitation, it is possible to limitthe values of w1 and w2 so that 1 is always selected for such values andgenerate the optimum values of only c1 and c2, which are DC components,in the linear prediction coefficient generation unit 110. Taking thestructure of FIG. 11 as an example, (1, c_(—)1) for ref1 and (1, c_(—)2)for ref2 are coded as parameter sets. In this case, the pixel value P(i)of the predictive image is calculated by the following equation in whichw1_(—)1 and w2_(—)2 in the equation 1a are substituted for 1.P(i)=(Q1(i)+Q2(i))/pow(2, d)+(c _(—)1+c _(—)2)/2

-   -   (where pow(2, d) indicates the “d”th power of 2)

Accordingly, it is possible to significantly reduce the processingamount for linear prediction even in exactly the same structure as thatof the first embodiment. It is also possible to significantly simplifythe method for determining coefficients because necessary coefficientsare only c_(—)1 and c_(—)2.

FIG. 24 shows an example where a flag sft_flg, indicating whether or notit is possible to perform linear prediction using only bit shiftoperations, and a flag dc_flg, indicating whether or not it is possibleto perform linear prediction using only c that is a DC component, aredescribed in picture common information in a coded stream of a picture.A decoding apparatus can decode the picture without reference to theseflags. However, referring to these flags, it is possible to performdecoding in the structure suitable for linear prediction using only bitshift operations or decoding in the structure suitable for linearprediction using only a DC component, so these flags could be veryimportant information depending on the structure of the decodingapparatus.

Although FIG. 24 shows the example where a coded stream is made up of apicture common information area and slice data areas, the picture commoninformation area and the slice data areas can be handled as separatecoded streams in exactly the same manner as one coded stream. Also, inthe example of FIG. 24, sft_flg and dc_flg are described in the picturecommon information area, but they can be handled in exactly the samemanner even if they are described in the sequence common informationarea or another independent common information area. In addition, it ispossible not only to use both of these two flags sft_flg and dc_flg atthe same time, but also to use either one of them, and they can behandled in the same manner in the latter case.

Third Embodiment

The moving picture coding method in the third embodiment of the presentinvention will be explained. Since the structure of the codingapparatus, the flow of coding processing and the reference indexassignment method are exactly same as those in the first embodiment, theexplanation thereof is not repeated here.

As explained in the background art, there is a method for generating apredictive image using a predetermined fixed equation such as theequation 2a and the equation 2b, unlike the first and second embodimentsin which a predictive image is generated using a prediction equationobtained from weighting coefficient sets of linear predictioncoefficients. This conventional method has an advantage that a dataamount for coding can be reduced because there is no need to code andtransmit the weighting coefficient sets used for generating thepredictive image. Also, the processing amount for linear prediction canbe significantly reduced because the equations for linear prediction aresimple. However, the method using the fixed equations has a problem thatprediction accuracy is lowered because there are only two selectablelinear prediction equations 2a and 2b.

So, in the present embodiment, the equations 8a and 8b are used insteadof the equations 2a and 2b. These equations 8a and 8b are obtained byadding C1 and C2 to the equations 2a and 2b. Since only the number ofadditions increases for the operation, there is little increase inprocessing amount, compared with the original equations 2a and 2b. Inthe following equations, Q1(i) and Q2(i) are pixel values ofreferredblocks, P(i) is a pixel value of a predictive image of a current blockto be coded, and C1 and C2 are linear prediction coefficients given by aselected weighting coefficient set.P(i)=2×(Q1(i)+C1)−(Q2(i)+C2)  Equation 8aP(i)=(Q1(i)+C1+Q2(i)+C2)/2  Equation 8b

The equations 8a and 8b are prediction equations for generating apredictive image with reference to two pictures at a time, but whengenerating a predictive image with reference to only one picture, theequation 9 is used instead of the equation 3 or 4 as explained in theabove embodiments.P(i)=Q1(i)+C1  Equation 9

The weighting coefficient sets for using this method are only (C1) forref1 and (C2) for ref2. Therefore, an example of a coded stream of apicture obtained using this method is as shown in FIG. 14. In the sliceheader area, the weighting coefficient sets for linear prediction(pset0, pset1, pset2, . . . ) are described for ref1 and ref2separately, and each of the weighting coefficient sets includes only C.Similarly, FIG. 15 shows an example of weighting coefficient setsincluded in the slice header area. Differently from FIG. 7, each of theweighting coefficient sets in FIG. 15 includes only C.

FIG. 16 is a block diagram showing the functional structure forgenerating a predictive image via the linear prediction coefficientgeneration unit 110, the linear prediction coefficient storage unit 109and the motion compensation coding unit 107 in FIG. 1.

A predictive image is generated via the linear prediction coefficientgeneration unit 110, the linear prediction coefficient storage unit 109g, the linear prediction coefficient storage unit 109 h and the linearprediction operation unit 107 i.

The weighting coefficient sets generated by the linear predictioncoefficient generation unit 110 are stored in the linear predictioncoefficient storage unit 109 g and the linear prediction coefficientstorage unit 109 h. Using the first reference index ref1 and the secondreference index ref2 determined by the motion estimation processing, theweighting coefficient sets (C1) and (C2) having one element respectivelyare obtained from the linear prediction coefficient storage units 109 gand 109 h. These values are inputted to the linear prediction operationunit 107 i, where linear prediction is performed on them using theequations 8a and 8b, and then the predictive image is generated.

Similarly, when linear prediction is performed with reference to onlyone picture, either one of the weighting coefficient sets (C1) and (C2)is obtained using only one of ref1 and ref2 in FIG. 16, linearprediction is performed using the equation 9, and then the predictiveimage is generated.

Note that the linear prediction coefficient generation unit 110generates the weighting coefficient sets (C1) and (C2) by examining thecharacteristics of a picture, creates a predictive image using themethod as explained in FIG. 16, and then determines a combination of thetwo reference indices ref1 and ref2 so as to minimize the predictionerror.

Since the present embodiment requires only one parameter to be used foreach of ref1 and ref2, the coding apparatus can determine the values ofparameters easily and further the data amount to be described in thecoded stream can be reduced. Also, since the linear prediction equationsdo not require complicated operations such as multiplications, theoperation amount can also be minimized. In addition, the use of thecoefficients C1 and C2 allows dramatic improvement of the low predictionaccuracy, which is considered as a disadvantage of the conventionalmethod of using a fixed equation.

Note that it is possible to use the linear prediction method asexplained in the present embodiment, regardless of whether a pluralityof reference indices can refer to the same picture or not.

Fourth Embodiment

The moving picture coding method in the fourth embodiment of the presentinvention will be explained. Since the structure of the codingapparatus, the flow of coding processing and the reference indexassignment method are exactly same as those in the first embodiment, theexplanation thereof is not repeated here.

Display order information indicating display time or the alternativethereto as well as a picture number are assigned to each picture. FIG.39 is a diagram showing an example of picture numbers and thecorresponding display order information. Certain values are assigned tothe display order information according to display order. In thisexample, the value which increases one by one for each picture is used.A method for generating values of coefficients used in an equation forlinear prediction using this display order information will be explainedin the fourth embodiment.

In the first embodiment, linear prediction is performed for each pixelusing the equation 1, the equation 3 or the equation 4 when generating apredictive image in motion compensation. However, since the linearprediction requires data of coefficients, such coefficient data isdescribed in slice header areas in a coded stream as weightingcoefficient sets to be used for creation of the predictive image.Although this method achieves high coding efficiency, it requiresadditional processing for creating data of the weighting coefficientsets and causes increase in bit amount because the weighting coefficientsets are described in the coded stream.

So, it is also possible to perform linear prediction using the equation10, equation 11a and the equation 12a instead of the equation 1. Usingthese equations, the weighting coefficients can be determined based ononly the display order information of each reference picture, so thereis no need to code the weighting coefficient sets separately.

In the following equations, Q1(i) and Q2(i) are pixel values of referredblocks, P(i) is a pixel value of a predictive image of a current blockto be coded, V0 and V1 are weighting coefficients, T0 is the displayorder information of the current picture to be coded, T1 is the displayorder information of the picture designated by the first referenceindex, and T2 is the display order information of the picture designatedby the second reference index.P(i)=V1×Q1(i)+V2×Q2(i)  Equation 10V1=(T2−T0)/(T2−T1)  Equation 11aV2=(T0−T1)/(T2−T1)  Equation 12a

When it is assumed, as an example, that the current picture to be codedis No. 16, the picture designated by the first reference index is No.11, and the picture designated by the second reference index is No. 10,the display order information of respective pictures are 15, 13 and 10,and thus the following linear prediction equations are determined.V1=(10-15)/(10-13)=5/3V2=(15-13)/(10-13)=−2/3P(i)=5/3×Q1(i)−2/3×Q2(i)

Compared with the method for performing linear prediction using theweighting coefficient sets in the equation 1, the above equations havelower flexibility of coefficient values, and thus it can be said that itis impossible to create the optimum predictive image. However, comparedwith the method of switching the two fixed equations 2a and 2b dependingon the positional relationship between two reference pictures, the aboveequations are more efficient linear prediction equations.

When both the first reference index and the second reference index referto the same picture, the equation 11a and the equation 12a do not holdbecause T1 becomes equal to T2. So, when the two reference pictures havethe same display order information, linear prediction shall be performedusing ½ as the values of V1 and V2. In that case, the linear predictionequations are as follows.V1= 1/2V2= 1/2P(i)=½×Q1(i)+½Q2(i)

Also, the equations 11a and 12a do not hold because T1 becomes equal toT2 when the first reference index and the second reference index referto different pictures but these pictures have the same display orderinformation. When the two reference pictures have the same display orderinformation as mentioned above, linear prediction shall be performedusing ½ as the values of V1 and V2.

As described above, when two reference pictures have the same displayorder information, it is possible to use a predetermined value as acoefficient. Such predetermined coefficient may be one having the sameweight like ½ as shown in the above example.

By the way, use of the equation 10 in the above embodiments requiresmultiplications and divisions for linear prediction. Since the linearprediction operation using the equation 10 is the operation for all thepixels in a current block to be coded, addition of multiplicationscauses significant increase in the processing amount.

So, approximation of V1 and V2 to the powers of 2 like the case of thesecond embodiment makes it possible to perform linear predictionoperation using only shift operations, and thus to achieve reduction ofthe processing amount. As the linear prediction equations for that case,the equations 11b and 12b are used instead of the equations 11a and 12a.In the following equations, v1 and v2 shall be integers.V1=±pow(2, v1)=apx((T2−T0)/(T2−T1))  Equation 11bV2=±pow(2, v2)=apx((T0−T1)/(T2−T1))  Equation 12b

-   -   (where pow(2, v1) indicates the “v1”th power of 2 and pow(2, v2)        indicates the “v2”th power of 2)    -   (where=apx( ) indicates that the value in ( ) shall be        approximated to the left-hand value)

Note that it is also possible to use the equations 11c and 12c insteadof the equations 11a and 12a, where v1 is an integer.V1=±pow(2, v1)=apx((T2−T0)/(T2−T1))  Equation 11cV2=1−V1  Equation 12c

-   -   (where pow(2, v1) indicates the “v1”th power of 2)    -   (where=apx( ) indicates that the value in ( ) shall be        approximated to the left-hand value)

Note that it is also possible to use the equations 11d and 12d insteadof the equations 11a and 12a, where v1 is an integer.V1=1−V2  Equation 11dV2=±pow(2, v2)=apx((T0−T1)/(T2−T1))  Equation 12d

-   -   (where pow(2, v2) indicates the “v2”th power of 2)    -   (where=apx( ) indicates that the value in ( ) shall be        approximated to the left-hand value)

Note that the value of V1 and V2 approximated to the power of 2 shallbe, taking the equation 1b as an example, the value of ±pow(2, v1)obtained when the values of ±pow(2, v1) and (T2−T0)/(T2−T1) are mostlyapproximated to each other as the value of v1 changes one by one.

For example, in FIG. 39, when a current picture to be coded is No. 16,the picture designated by the first reference index is No. 11, and thepicture designated by the second reference index is No. 10, the displayorder information of the respective pictures are 15, 13 and 10, so(T2−T0)/(T2−T1) and ±pow(2, v1) are as follows.

-   -   (T2−T0)/(T2−T1)=(10-15)/(10-13)=5/3    -   +pow(2, 0)=1    -   +pow(2, 1)=2

Since 5/3 is close to 2 rather than 1, V1=2 is obtained as a result ofapproximation.

As another approximation method, it is also possible to switch betweenround-up approximation and round-down approximation depending on therelationship between two values of display order information T1 and T2.

In that case, round-up approximation is performed on both V1 and V2 whenT1 is later than T2, and round-down approximation is performed on bothV1 and V2 when T1 is earlier than T2. It is also possible, in a reversemanner, to perform round-down approximation on both V1 and V2 when T1 islater than T2, and round-up approximation on both V1 and V2 when T1 isearlier than T2.

As another approximation method using display order information,round-up approximation is performed in an equation for V1 and round-downapproximation is performed in an equation for V2 when T1 is later thanT2. As a result, the difference between the values of the twocoefficients becomes large, it is likely that values suitable forextrapolation are obtained. On the contrary, when T1 is earlier than T2,the value in the equation for V1 and the value in the equation for V2are compared, and then round-up approximation is performed on thesmaller value and round-down approximation is performed on the largervalue. As a result, the difference between the values of the twocoefficients becomes small, so it is likely that values suitable forinterpolation are obtained.

For example, in FIG. 39, when a current picture to be coded is No. 16,the picture designated by the first reference index is No. 11, and thepicture designated by the second reference index is No. 10, the displayorder information of the respective pictures are 15, 13 and 10. Since T1is later than T2, round-up approximation is performed on the equationfor V1 and round-down approximation is performed on the equation for V2.As a result, the equations 11b and 12b are calculated as follows.(T2−T0)/(T2−T1)=(10-15)/(10-13)=5/3+pow(2, 0)=1+pow(2, 1)=2  (1) Equation 11b

As a result of round-up approximation, V1=2 is obtained.(T0−T1)/(T2−T1)=(15-13)/(10-13)=−2/3−pow(2, 0)=−1−pow(2, −1)=−½  (2) Equation 12b

As a result of round-down approximation, V2=−1 is obtained. Note thatalthough the equation 10 is only one equation for liner prediction inthe above embodiments, it is also possible to combine this method withthe linear prediction method by use of the two fixed equations 2a and 2bas explained in the background art. In that case, the equation 10 isused instead of the equation 2a, and the equation 2b is used as it is.In other words, the equation 10 is used when the picture designated bythe first reference index appears behind the picture designated by thesecond reference index in display order, whereas the equation 2b is usedin other cases.

It is also possible, on the contrary, to use the equation 10 instead ofthe equation 2b and use the equation 2a as it is. In other words, theequation 2a is used when the picture designated by the first referenceindex appears behind the picture designated by the second referenceindex, and the equation 10 is used in other cases. However, when tworeference pictures have the same display order information, linearprediction is performed using ½ as the values of V1 and V2.

It is also possible to describe only the coefficient C in the sliceheader areas to be used for linear prediction, in the same manner as theconcept of the third embodiment. In that case, the equation 13 is usedinstead of the equation 10. V1 and V2 are obtained in the same manner asthe above embodiments.P(i)=V1×(Q1(i)+C1)+V2×(Q2(i)+C2)  Equation 13

The processing for generating coefficients C is needed, and further thecoefficients C needs to be coded into the slice header area, but use ofC1 and C2 allows more accurate linear prediction even if the accuracy ofV1 and V2 is low. This is effective particularly in the case where V1and V2 are approximated to the powers of 2 for performing linearprediction.

Note that using the equation 13, the linear prediction can be performedin the same manner in both cases where one reference index is assignedto one picture and a plurality of reference indices are assigned to onepicture.

In the calculation of the values in each of the equations 11a, 12a, 11b,12b, 11c, 12c, 11d and 12d, the combinations of available values arelimited to some extent in every slice. So, only one operation is enoughfor coding a slice, unlike the equation 10 or the equation 13 in whichthe operation needs to be performed for all the pixels in a currentblock to be coded, and thus there seems to be little influence on theentire processing amount.

Note that the display order information in the present embodiment is notlimited to display order, but may be the actual display time, or theorder of respective pictures starting from a predetermined picture, withits value increasing as the lapse of display time.

Fifth Embodiment

The moving picture coding method in the fifth embodiment of the presentinvention will be explained. Since the structure of the codingapparatus, the flow of coding processing and the reference indexassignment method are exactly same as those in the first embodiment, theexplanation thereof is not repeated here.

In the conventional method, it is possible to switch between generationof a predictive image by use of fixed equations and generation of apredictive image by use of weighting coefficient sets of linearprediction coefficients, using flags described in a picture commoninformation area in a coded stream, if necessary.

In the present embodiment, another method for switching the variouslinear prediction methods as explained in the above first to fourthembodiments using flags will be explained.

FIG. 17A shows the structure used for the case where five flags (p_flag,c_flag, d_flag, t_flag, s_flag) for controlling the above switching aredescribed in the slice header area in the coded stream.

As shown in FIG. 17B, p_flag is a flag indicating whether weightingcoefficients have been coded or not. c_flag is a flag indicating whetheronly the data of the parameter C (C1 and C2), out of the parameters forref1 and ref2, has been coded or not. t_flag is a flag indicatingwhether weighting coefficients for linear prediction are to be generatedor not using the display order information of the reference pictures.And s_flag is a flag indicating whether the weighting coefficients forlinear prediction are to be approximated to the powers of 2 or not forthe calculation by use of shift operations.

Furthermore, d_flag is a flag indicating whether or not to switch twopredetermined fixed equations like the equations 2a and 2b depending onthe temporal positional relationship between the picture designated byref1 and the picture designated by ref2, when linear prediction isperformed using such two fixed equations. To be more specific, when thisflag indicates the switching of the equations, the equation 2a is usedin the case where the picture designated by ref1 is later than thepicture designated by ref2 in display order, and the equation 2b is usedin other cases for performing linear prediction, as is the case with theconventional method. On the other hand, when this flag indicates noswitching of the equations, the equation 2b is always used forperforming linear prediction, regardless of the positional relationshipbetween the picture designated by ref1 and the picture designated byref2.

Note that even if the equation 2a is used instead of the equation 2b asan equation to be used without switching, the equation 2a can be handledin the same manner as the equation 2b.

In the coding apparatus as shown in FIG. 1, the motion compensationcoding unit 107 determines whether or not to code the data concerningthe weighting coefficient sets on a per-slice basis, outputs theinformation of the flag p_flag to the coded stream generation unit 103based on the determination, and describes the information in the codedstream as shown in FIG. 17A. As a result, it is possible to use theweighting coefficient sets in a higher performance apparatus forperforming linear prediction, and not to use the weighting coefficientsets in a lower performance apparatus for performing linear prediction.

Similarly, in the coding apparatuses shown in FIG. 1, the motioncompensation coding unit 107 determines, on a per-slice basis, whetheror not to code only the data concerning the parameter C (C1 and C2)corresponding to the DC components of image data, outputs theinformation of the flag c_flag to the coded stream generation unit 103based on the determination, and describes the information in the codedstream as shown in FIG. 17A. As a result, it is possible to use all theweighting coefficient sets in a higher performance apparatus forperforming linear prediction, and to use only the DC components in alower performance apparatus for performing linear prediction.

Similarly, in the coding apparatus as shown in FIG. 1, when linearprediction is performed using fixed equations, the motion compensationcoding unit 107 determines, on a per-slice basis, whether or not toperform coding by switching two equations, outputs the information ofthe flag d_flag to the coded stream generation unit 103 based on thedetermination, and describes the information in the coded stream asshown in FIG. 17A. As a result, it is possible to use either one of thefixed equations for linear prediction in the case where there is littletemporal change in brightness of a picture, and to switch between thetwo fixed equations for linear prediction in the case where thebrightness of the picture changes as time goes by.

Similarly, in the coding apparatus as shown in FIG. 1, the motioncompensation coding unit 107 determines, on a per-slice basis, whetheror not to generate coefficients for linear prediction using the displayorder information of the reference pictures, outputs the information ofthe flag t_flag to the coded stream generation unit 103 based on thedetermination, and describes the information in the coded stream asshown in FIG. 17A. As a result, it is possible to code the weightingcoefficient sets for linear prediction in the case where there is enoughspace to be coded in the coded stream, and to generate the coefficientsfrom the display order information for linear prediction in the casewhere there is no more space to be coded.

Similarly, in the coding apparatus as shown in FIG. 1, the motioncompensation coding unit 107 determines, on a per-slice basis, whetheror not to approximate the coefficients for linear prediction to thepowers of 2 so as to allow the calculation of these coefficients byshift operations, outputs the information of the flag s_flag to thecoded stream generation unit 103 based on the determination, anddescribes the information in the coded stream as shown in FIG. 17A. As aresult, it is possible to use the weighting coefficients without beingapproximated in a higher performance apparatus for performing linearprediction, and to use the weighting coefficients with beingapproximated to the powers of 2 in a lower performance apparatus forperforming linear prediction which can be realized only by shiftoperations.

For example, the case of (1) (p, c, d, t, s_flag)=(1, 0, 0, 0, 1) showsthat all the weighting coefficient sets are coded and linear predictionis performed by only shift operations by representing the coefficientsas the powers of 2 as explained in the second embodiment so as togenerate a predictive image.

The case of (2) (p, c, d, t, s_flag)=(1, 1, 1, 0, 0) shows that only thedata concerning the parameter C (C1 and C2) is coded, the method forgenerating a predictive image by adding the coefficient C to the fixedequations as explained in the third embodiment is used, and further thetwo fixed equations are switched for use.

In the case of (3) (p, c, d, t, s_flag)=(0, 0, 0, 0, 0), no weightingcoefficient set is coded. In other words, it shows that the method forgenerating a predictive image using only the equation 2b, out of thefixed equations in the conventional method, is used.

The case of (4) (p, c, d, t, s_flag)=(0, 0, 1, 1, 1) shows that noweighting coefficient set is coded, but the weighting coefficients aregenerated from the display order information of the reference picturesand linear prediction is performed by only shift operations byapproximating the generated coefficients to the powers of 2, and thentwo fixed equations are switched for use so as to generate a predictiveimage.

Note that in the present embodiment, the determinations are made usingfive flags (p_flag, c_flag, d_flag, t_flag, s_flag), each of which is 1bit, but it is also possible to represent the determinations with onlyone flag of 5 bits, instead of these five flags. In that case, coding byuse of variable length coding is available, not by representation with 5bits.

Note that in the present embodiment, all the five flags (p_flag, c_flag,d_flag, t_flag, s_flag), each of which is 1 bit, are used, but the samecan be applied to the case where linear prediction method is switchedonly using some of these flags. In that case, the flags necessary forthe linear prediction are only coded and described, out of the flags asshown in FIG. 17A.

The conventional method allows switching, on a picture-by-picture basis,between the predictive image generation method by use of fixed equationsand the predictive image generation method by use of weightingcoefficient sets of linear prediction coefficients, by providing a flagfor switching these methods in a picture common information area in acoded stream. However, in the conventional method, the predictive imagegeneration method can only be switched on a picture-by-picture basis.

As explained above, the present embodiment allows switching of thepredictive image generation method in each slice being a subdivision ofa picture by providing this switching flag in a slice header in a codedstream. Therefore, it is possible to generate a predictive image usingthe weighting coefficients in a slice having complex images and togenerate a predictive image using fixed equations in a slice havingsimple images, and thus it is possible to improve the image qualitywhile minimizing the increase in processing amount.

Note that in the present embodiment, five flags (p_flag, c_flag, d_flag,t_flag, s_flag) are described in a slice header area for determiningwhether the coefficients are coded or not for each slice, but it is alsopossible to switch the determination on a picture-by-picture basis bydescribing these flags in the picture common information area. Inaddition, it is also possible to generate a predictive image using theoptimum method on a block-by-block basis by providing the switching flagon each block being a subdivision of a slice.

Note that the display order information in the present embodiment is notlimited to display order, but may be the actual display time, or theorder of respective pictures starting from a predetermined picture, withits value increasing as the lapse of display time.

Sixth Embodiment

FIG. 2 is a block diagram showing a structure of a moving picturedecoding apparatus in the sixth embodiment of the present invention.Using this block diagram as shown in FIG. 2, the moving picture decodingmethod executed by this moving picture decoding apparatus will beexplained in the following order: (1) an overview of decoding and (2) amethod for assigning reference indices, and (3) a method for generatinga predictive image. Here, it is assumed that a coded stream which isgenerated by the moving picture coding method in the first embodiment isinputted to the present decoding apparatus.

(1) Overview of Decoding

First, the coded stream analysis unit 201 extracts the data sequence ofweighting coefficient sets for linear prediction and the commandsequence for assignment of reference indices from the slice header area,and various information such as the reference index information, themotion vector information and the coded prediction error data from thecoded block information area. FIG. 6 shows the above-mentioned variouscoded information in the coded stream.

The data sequence of the weighting coefficient sets for linearprediction extracted by the coded stream analysis unit 201 is outputtedto a linear prediction coefficient storage unit 206, the commandsequence for reference index assignment is outputted to a referenceindex/picture number conversion unit 207, the reference indices areoutputted to a motion compensation decoding unit 204, the motion vectorinformation is outputted to a motion vector storage unit 205, and thecoded prediction error signal is outputted to a prediction errordecoding unit 202, respectively.

The prediction error decoding unit 202 performs image decodingprocessing such as inverse-quantization and inverse-frequency transformfor the inputted coded prediction error signal, and outputs a decodederror signal. The addition unit 208 adds the decoded prediction errorsignal and the predictive image signal outputted from the motioncompensation decoding unit 204 to generate a reconstructed image signal.The obtained reconstructed image signal is stored into a picture memory203 for use for reference in the following inter-picture prediction andoutput for display.

The motion compensation decoding unit 204 extracts an image area whichis most suitable as a predictive image from the reconstructed imagesignals stored in the picture memory 203, using the motion vectorsinputted from the motion vector storage unit 205 and the referenceindices inputted from the coded stream analysis unit 201. At this time,the reference index/picture number conversion unit 207 designates thereference pictures in the picture memory 203 based on the correspondencebetween the reference indices given from the coded stream analysis unit201 and the picture numbers.

The operation of the reference index/picture number conversion unit 207will be explained in detail in the section (2). Further, the motioncompensation decoding unit 204 performs pixel value conversionprocessing such as interpolation processing by linear prediction onpixel values in the extracted image area, so as to generate the finalpredictive image. The linear prediction coefficients used for thatprocessing are obtained from the data stored in the linear predictioncoefficient storage unit 206 using the reference indices as search keys.

This predictive image generation method will be explained in detail inthe section (3).

The decoded image generated through the above-mentioned series ofprocesses is stored in the picture memory 203, and outputted as apicture signal for display according to display timing.

The flow of operations in the case of inter-picture prediction decodinghas been described above, but a switch 209 switches betweeninter-picture prediction decoding and intra-picture prediction decoding.In the case of intra-picture decoding, a predictive image is notgenerated by motion compensation, but a decoded image is generated bygenerating a predictive image of a current area to be decoded from adecoded area in the same picture and adding the predictive image. Thedecoded image is stored in the picture memory 203, as is the case withthe inter-picture prediction decoding, and outputted as a picture signalfor display according to display timing.

(2) Method for Assigning Reference Indices

Next, a method for assigning reference indices in the referenceindex/picture number conversion unit 207 in FIG. 2 will be explainedusing FIG. 3 and FIG. 4.

FIG. 3 is a diagram for explaining a method for assigning two referenceindices to picture numbers. When there is a sequence of pictures orderedin display order, picture numbers are assigned in decoding order.Commands for assigning the reference indices to the picture numbers aredescribed in a header of a slice that is a subdivision of a picture, asthe unit of decoding, and thus the assignment thereof is updated everytime one slice is decoded. The command indicates, in series by thenumber of reference indices, the differential value between a picturenumber which is assigned a reference index currently and a picturenumber which is assigned a reference index immediately before thecurrent assignment.

Taking the first reference index in FIG. 3 as an example, since “−1” isgiven as a command first, 1 is subtracted from the picture number 16 ofthe current picture to be decoded and thus the reference index 0 isassigned to the picture number 15. Next, since “−4” is given, 4 issubtracted from the picture number 15 and thus the reference index 1 isassigned to the picture number 11. The following reference indices areassigned to respective picture numbers in the same manner. The sameapplies to the second reference indices.

According to the conventional reference index assignment method as shownin FIG. 34, all the reference indices are corresponded to respectivepicture numbers. On the other hand, in the example of FIG. 3, theassignment method is exactly same as the conventional method, but aplurality of reference indices are corresponded to the same picturenumber by changing the values of the commands.

FIG. 4 shows the result of the assignment of the reference indices. Thisdiagram shows that the first reference indices and the second referenceindices are assigned to respective picture numbers separately, but aplurality of reference indices are assigned to one picture number insome cases. In the decoding method of the present invention, it isassumed that a plurality of reference indices are assigned to at leastone picture number, like this example.

If the reference indices are used only for determination of referencepictures, the conventional method of one-to-one assignment of referenceindices to picture numbers is the most efficient method. However, in thecase where a weighting coefficient set of linear prediction coefficientsis selected for generation of a predictive image using referenceindices, the same linear prediction coefficients have to be used for allthe blocks having the same reference pictures, so there is an extremelyhigh possibility that the optimum predictive image cannot be generated.So, if it is possible to assign a plurality of reference indices to onepicture number as in the case of the present invention, the optimumweighting coefficient set of linear prediction coefficients can beselected from among a plurality of candidate sets for each block even ifall the blocks have the same reference picture, and thus the predictiveimage with its prediction accuracy being higher can be generated.

Note that, the above description shows the case where the picturenumbers are given assuming that all the reference pictures are stored ina reference memory. However, since a current picture is given a picturenumber which is larger by one than the number of a picture which hasbeen coded immediately before the current picture, only when the currentpicture which has been coded lastly is stored, so the continuity of thepicture numbers are maintained in the reference memory even if somepictures are not stored, and thus the above-mentioned method can be usedwithout change.

(3) Method for Generating Predictive Image

Next, the predictive image generation method in the motion compensationdecoding unit 204 in FIG. 2 will be explained using FIG. 5. Although thepredictive image generation method by linear prediction is exactly thesame as the conventional method, flexibility in selection of linearprediction coefficients is increased because a plurality of referenceindex numbers can be corresponded to the same picture.

The picture B16 is a current B-picture to be decoded, and the blocksBL01 and BL02 are current blocks to be decoded which belong to theB-picture. The picture P11 and the picture B15 are used as the firstreference picture and the second reference picture for BL01, and thepredictive image is generated with reference to the blocks BL11 and BL21belonging to the pictures P11 and B15 respectively. In the same manner,the picture P11 and the picture B15 are used as the first referencepicture and the second reference picture for BL02, and the predictiveimage is generated with reference to the blocks BL12 and BL22respectively.

Although both BL01 and BL02 refer to the same pictures as their firstreference picture and the second reference picture, it is possible toassign different values to the first reference index ref1 and the secondreference index ref2 for BL01 and BL02 respectively by using thereference index assignment method as explained in the above (2). TakingFIG. 4 as an example, 1 and 3 are assigned to the first reference indexcorresponding to the picture number 11, whereas 1 and 6 are assigned tothe second reference index corresponding to the picture number 15. As aresult, four combinations of these reference indices, (ref1, ref2)=(1,1), (1, 6), (3, 1) and (3, 6) are supposed, and thus it is possible toselect the combination for deriving the optimum weighting coefficientset on a per-block basis from among these combinations. In FIG. 5,ref1=1 and ref2=1 are assigned for BL01, and ref1=3 and ref2=6 areassigned for BL02, for example.

According to the conventional reference assignment method as shown inFIG. 35, only one combination of (ref1, ref2)=(1, 1) can be selected forboth BL01 and BL02 in the case of FIG. 5, and thus only one weightingcoefficient set of linear prediction coefficients can be selected, too.On the other hand, according to the present invention, four options areavailable, and it can be said that the possibility for selection of theoptimum weighting coefficient set is increased.

A coded stream of one picture is made up of a picture common informationarea and a plurality of slice data areas. FIG. 6 shows the structure ofthe slice data area in the coded stream. The slice data area is furthermade up of a slice header area and a plurality of block data areas. Thisdiagram shows each block area corresponding to BL01 and BL02 in FIG. 5as an example of the block data area.

“ref1” and “ref2” included in BL01 designate the first reference indexand the second reference index respectively indicating two picturesreferred to by the block BL01. Also, in the slice header area, data forgiving the weighting coefficient sets for the above-mentioned linearprediction (pset0, pset1, pset2, . . . ) is described for ref1 and ref2respectively. In this area, “pset” s of the number equivalent to thenumber of the reference indices as explained in (2) can be set. To bemore specific, in the case where ten reference indices from 0 to 9 areused as each of the first reference index and the second referenceindex, ten “pset” s from 0 to 9 can be set for each of ref1 and ref2.

FIG. 7 shows an example of tables of the weighting coefficient setsincluded in the slice header area. Each data indicated by an identifierpset has four values w1, w2, c and d, and these tables are structured sothat these values can be directly referred to by the values of ref1 andref2. Also, command sequences idx_cmd1 and idx_cmd2 for assigning thereference indices to the picture numbers are described in the sliceheader area.

Using ref1 and ref2 described in BL01 in FIG. 6, one set of weightingcoefficients is selected from each of the tables for ref1 and ref2 inFIG. 7. By performing linear prediction on the pixel values of thereference images using these two sets of weighting coefficients, apredictive image is generated.

Here, the flow of the processing from determination of weightingcoefficient sets until generation of a predictive image will beexplained.

FIG. 18 is a functional block diagram showing a functional structure forgenerating a predictive image in the linear prediction coefficientstorage unit 206 and the motion compensation decoding unit 204.

A predictive image is generated via the linear prediction coefficientstorage unit 206 a, the linear prediction coefficient storage unit 206b, the average calculation unit 204 a and the linear predictionoperation unit 204 b.

The average calculation unit 204 a obtains, from the linear predictioncoefficient storage unit 206 a, one set of weighting coefficients(w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected by ref1 outputted from thecoded stream analysis unit 201, and obtains, from the linear predictioncoefficient storage unit 206 b, one set of weighting coefficients(w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected by ref2 outputted from thecoded stream analysis unit 201.

The average calculation unit 204 a calculates the average, forrespective parameters, of the weighting coefficient sets obtained fromthe linear prediction coefficient storage units 206 a and 206 brespectively so as to output the averages to the linear predictionoperation unit 204 b as the weighting coefficient set (w1, w2, c, d)which is actually used for linear prediction. The linear predictionoperation unit 204 b calculates the predictive image using the equation1 based on the weighting coefficient set (w1, w2, c, d) for output.

FIG. 19 is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient storage unit 206 a, thelinear prediction coefficient storage unit 206 b, the linear predictionoperation unit 204 c, the linear prediction operation unit 204 d and theaverage calculation unit 204 e.

The linear prediction operation unit 204 c obtains, from the linearprediction coefficient storage unit 206 a, one set of weightingcoefficients (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected by ref1outputted from the coded stream analysis unit 201, and calculates thepredictive image using the equation 1 based on the weighting coefficientset for output to the average calculation unit 204 e.

In the same manner, the linear prediction operation unit 204 d obtains,from the linear prediction coefficient storage unit 206 b, one set ofweighting coefficients (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected byref2 outputted from the coded stream analysis unit 201, and calculatesthe predictive image using the equation 1 based on the weightingcoefficient set for output to the average calculation unit 204 e.

The average calculation unit 204 e calculates the average, forrespective pixel values, of the predictive images which are outputtedfrom the linear prediction operation unit 204 c and the linearprediction operation unit 204 d respectively so as to generate the finalpredictive image for output.

FIG. 20A is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient storage unit 206 c, thelinear prediction storage unit 206 d, the average calculation unit 204f, and the linear prediction operation unit 204 g.

The average calculation unit 204 f obtains, from the linear predictioncoefficient storage unit 206 c, the parameters of c_(—)1 and d_(—)1 inone set of weighting coefficients (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1)selected by ref1 outputted from the coded stream analysis unit 201, andsimilarly, obtains, from the linear prediction coefficient storage unit206 d, the parameters of c_(—)2 and d_(—)2 in one set of weightingcoefficients (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) selected by ref2outputted from the coded stream analysis unit 201. The averagecalculation unit 204 f calculates the average of c_(—)1 and c_(—)2 andthe average of d_(—)1 and d_(—)2 obtained from the linear predictioncoefficient storage unit 206 c and the linear prediction coefficientstorage unit 206 d so as to obtain c and d for output to the linearprediction operation unit 204 g.

Also, the linear prediction operation unit 204 g obtains the parameterof w1_(—)1 in the above-mentioned weighting coefficient set (w1_(—)1,w2_(—)1, c_(—)1, d_(—)1) from the linear prediction coefficient storageunit 206 c, obtains the parameter of w2_(—)2 in the above-mentionedweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) from thelinear prediction coefficient storage unit 206 d, and obtains c and dwhich are the averages calculated by the average calculation unit 204 f,and then calculates the predictive image using the equation 1 foroutput.

To be more specific, when determining the weighting coefficient set (w1,w2, c, d) which is actually used for linear prediction from among theweighting coefficient set (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) obtainedfrom the linear prediction coefficient storage unit 206 c and theweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) obtainedfrom the linear prediction coefficient storage unit 206 d, the linearprediction operation unit 204 g uses the following rule.

-   -   w1=w1_(—)1    -   w2=w2_(—)2    -   c=(average of c_(—)1 and c_(—)2)    -   d=(average of d_(—)1 and d_(—)2)

FIG. 20B is a functional block diagram showing another functionalstructure for generating a predictive image. A predictive image isgenerated via the linear prediction coefficient storage unit 206 e, thelinear prediction coefficient storage unit 206 f and the linearprediction operation unit 204 h.

The linear prediction operation unit 204 h obtains, from the linearprediction coefficient storage unit 206 e, the parameters of w1_(—)1,c_(—)1 and d_(—)1 which are a part of one set of weighting coefficients(w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) selected by ref1 outputted from thecoded stream analysis unit 201, and similarly, obtains, from the linearprediction coefficient storage unit 206 f, the parameter of w2_(—)2which is a part of one set of weighting coefficients (w1_(—)2, w2_(—)2,c_(—)2, d_(—)2) selected by ref2 outputted from the coded streamanalysis unit 201. The linear prediction operation unit 204 h calculatesthe predictive image using the equation 1 based on w1_(—)1, c_(—)1,d_(—)1, w2_(—)2 obtained from the linear prediction coefficient storageunit 206 e and the linear prediction coefficient storage unit 206 f foroutput.

To be more specific, when determining the weighting coefficient set (w1,w2, c, d) which is actually used for linear prediction from among theweighting coefficient set (w1_(—)1, w2_(—)1, c_(—)1, d_(—)1) obtainedfrom the linear prediction coefficient storage unit 206 e and theweighting coefficient set (w1_(—)2, w2_(—)2, c_(—)2, d_(—)2) obtainedfrom the linear prediction coefficient storage unit 206 f, the linearprediction operation unit 204 h uses the following rule.

-   -   w1=w1_(—)1    -   w2=w2_(—)2    -   c=c_(—)1    -   d=d_(—)1

Furthermore, it is possible to use one or more parameters out of w1, w2,c and d as fixed values. FIG. 21 is a functional block diagram in thecase where only “d” is used as a fixed value for the functionalstructure in FIG. 20A. A predictive image is generated via the linearprediction coefficient storage unit 206 g, the linear predictioncoefficient storage unit 206 h, the average calculation unit 204 i andthe linear prediction operation unit 204 j.

The coefficients selected by the first reference index ref1 from thelinear prediction coefficient storage unit 206 g are only (w1_(—)1,c_(—)1), and the coefficients selected by the second reference index ref2 from the linear prediction coefficient storage unit 206 h are only(w2_(—)2, c_(—)2). The average calculation unit 204 i calculates theaverage of c_(—)1and c_(—)2 obtained from the linear predictioncoefficient storage unit 206 g and the linear prediction coefficientstorage unit 206 h so as to obtain “c”, and outputs it to the linearprediction operation unit 204 j.

The linear prediction operation unit 204 j obtains the parameter ofw1_(—)1 from the linear prediction coefficient storage unit 206 g,obtains the parameter of w2_(—)2 from the linear prediction coefficientstorage unit 206 h, and obtains the parameter of c from the averagecalculation unit 204 i, and then calculates the predictive image using apredetermined fixed value as a parameter of d and the equation 1 foroutput. In this case, it is also possible to modify the equation 1 intothe equation 1b for use as explained in the first embodiment.

It is possible to use one predetermined fixed value as a value of d inany cases, but in the case where the coding apparatus describes theabove fixed value in the slice header, that fixed value can be switchedon a per-slice basis by extracting it in the coded stream analysis unit201. Similarly, the fixed value can be switched on a per-picture basisor a per-sequence basis by describing it in the picture commoninformation area or the sequence common information area.

Note that the above-mentioned decoding methods are concerned with aB-picture having two reference pictures, but it is also possible toexecute the same processing in the single picture reference decodingmode for a P-picture or a B-picture having only one reference picture.In this case, using only one of the first reference index and the secondreference index, pset and idx_cmd for either ref1 or ref2 are onlydescribed in the slice header area included in the coded stream of FIG.6, according to the reference index described in the block data area. Inaddition, as a linear prediction method, the following equation 3 or theequation 4 is used instead of the equation 1 as explained in theconventional method.

Use of the equation 1 and the equation 3 requires four parameters w1,w2, c and d, but use of the equation 4 allows linear prediction by useof only three parameters w1, c and d. In other words, it is possible toreduce the number of data items of the weighting coefficient set to bedescribed in each slice header area to three, when either one of thefirst reference index and the second reference index is used for anentire picture like a P-picture.

If the equation 3 is used, it is possible to realize linear predictionwhich is available for both B-pictures and P-pictures adaptively withoutchange in structure. If the equation 4 is used, the data amount to bedescribed in the header area of a P-picture can be reduced, and thus itis possible to achieve reduction of processing amount because ofsimplified calculation. However, since the reference index assignmentmethod suggested by the present invention can directly be applied to anyof the above methods, a predictive image with higher prediction accuracycan be created, which is extremely effective in image decoding.

By the way, the pictures which are referred to in motion compensationare determined by designating the reference indices assigned torespective pictures. In that case, the maximum number of pictures whichare available for reference has been described in the picture commoninformation area in the coded stream.

FIG. 38 is a schematic diagram of a coded stream in which the maximumnumber of pictures which are available for reference is described. Asthis diagram shows, the maximum number of pictures for ref1 “Max_pic1”and the maximum number of pictures for ref2 “Max_pic2” are described inthe picture common information in the coded stream.

Information required for decoding is not the maximum number of realpictures themselves, but the maximum reference index value available fordesignating pictures.

Since one reference index is assigned to one picture in the conventionalmethod, above-mentioned description of the maximum number of picturescauses no contradiction. But the difference in number between referenceindices and pictures has a significant influence on the case where aplurality of reference indices are assigned to one picture number, likethe present invention.

As described above, command sequences idx_cmd1 and idx_cmd2 aredescribed in a coded stream in order to assign reference indices topicture numbers. The picture numbers and the reference indices arecorresponded to each other based on each command in these commandsequences idx_cmd1 and idx_cmd2. For that purpose, knowing the maximumreference index value shows that all the reference indices and picturenumbers have been corresponded to each other, namely, the end of thecommands in the command sequences idx_cmd1 and idx_cmd2.

Therefore, in the present embodiment, the maximum number of availablereference indices, instead of the maximum number of pictures in thebackground art, is described in the picture common information areabeing a header of the picture. Or, both of the maximum number ofpictures and the maximum number of reference indices are described.

FIG. 23 shows a picture common information area in a coded stream of apicture in which the maximum number of reference indices is described.In the picture common information area, the maximum number of referenceindices available for ref1 “Max_idx1” and the maximum number ofreference indices available for ref2 “Max_idx2” are described.

In FIG. 23, the maximum number of reference indices is described in thepicture common information, but it may be structured so that the maximumnumber of reference indices is described in the slice data area as wellas in the picture common information. For example, the maximum number ofreference indices required for each slice can be clearly described, inthe case where the maximum number of required reference indices for eachslice is significantly different from the maximum number thereofdescribed in the picture common information area, from slice to slice,for example, the maximum number of reference indices in a picture is 8,the maximum number of reference indices required for the slice 1 in thepicture is 8, and the maximum number of reference indices required forthe slice 2 is 4.

In other words, it may be structured so that the maximum number ofreference indices described in the picture common information is set tobe the default value common to all the slices in the picture, and themaximum number of reference indices required for each slice, which isdifferent from the default value, is described in the slice header.

Although FIG. 23 and FIG. 38 show the examples where a coded stream ismade up of a picture common information area and slice data areas, thepicture common information area and the slice data areas can be handledas separate coded streams in exactly the same manner as one codedstream.

Seventh Embodiment

The moving picture decoding method in the seventh embodiment of thepresent invention will be explained. Since the structure of the decodingapparatus, the flow of decoding processing and the reference indexassignment method are exactly the same as the sixth embodiment, theexplanation thereof is not repeated here.

In the sixth embodiment, linear prediction is performed on each pixelusing the equation 1, the equation 3 or the equation 4 for generating apredictive image in motion compensation. However, since all of theseequations include multiplications, which cause significant increase inprocessing amount in considering that these multiplications areperformed on all the pixels.

So, it is possible to use the equation 5 instead of the equation 1, theequation 6 instead of the equation 3, and the equation 7 instead of theequation 4. These equations allow calculations only by bit shiftoperations without using multiplications, and thus allows reduction ofprocessing amount.

As is the case with the sixth embodiment, the equation 5 is used forgenerating a predictive image with reference to two pictures at a time,and the equation 6 or the equation 7 is used for generating a predictiveimage with reference to only one picture. Since these equations requireidentifiers indicating plus and minus signs, weighting coefficient setsrequired for prediction operations are (sign1, m, sign2, n, c) for theequations 5 and 6, and (sign1, m, c) for the equation 7. “sing1” and“sign2” are parameters identifying the first and second plus and minussigns, respectively. Although the number of parameters is larger thanthat in the third embodiment, there is little increase in processingamount because both sign1 and sign2 can be represented by 1 bit.

Here, the flow of processing from determination of weighting coefficientsets until generation of a predictive image with reference to twopictures at a time using the equation 5 will be explained in detail.

First, the case where the functional structure for generation of apredictive image is that as shown in FIG. 18 will be explained. Theaverage calculation unit 204 a obtains the weighting coefficient set(sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1, c_(—)1) from the linearprediction coefficient storage unit 206 a. Also, the average calculationunit 204 a obtains the weighting coefficient set (sign1_(—)2, m_(—)2,sign2_(—)2, n_(—)2, c_(—)2) from the linear prediction coefficientstorage unit 206 b.

The average calculation unit 204 a calculates, for respectiveparameters, the average of the weighting coefficient sets obtained fromthe linear prediction coefficient storage unit 206 a and the linearprediction coefficient storage unit 206 b so as to consider the averageas the weighting coefficient set (sign1, m, sign2, n, c). The linearprediction operation unit 204 b calculates the predictive image usingthe equation 5 based on the weighting coefficient set (sign1, m, sign2,n, c) outputted from the average calculation unit 204 a.

Note that FIG. 18 shows the weighting coefficient set (w1_(—)1, w2_(—)1,c_(—)1, d_(—)1) and the like obtained from the linear predictioncoefficient storage unit 206 a and the like, which are calculated in thecase where the equation 1 is used as explained in the sixth embodiment,and does not show the parameters used in the case where the predictiveimage is obtained using the equation 5, but the parameters used in theformer case can be replaced by the parameters in the latter case as theyare. The same goes for the cases of FIG. 19 and FIG. 20 to be describedlater.

Next, the case where the functional structure for generation of apredictive image is that as shown in FIG. 19 will be explained. Thelinear prediction operation unit 204 c calculates a predictive image 1based on the weighting coefficient set (sign1_(—)1, m_(—)1, sign2_(—)1,n_(—)1, c_(—)1) obtained from the linear prediction coefficient storageunit 206 a. The linear prediction operation unit 204 d calculates apredictive image 2 based on the weighting coefficient set (sign1_(—)2,m_(—)2, sign2_(—)2, n_(—)2, c_(—)2) obtained from the linear predictioncoefficient storage unit 206 b. And the average calculation unit 204 ecalculates, for respective pixels, the average of the predictive imagescalculated by the linear prediction operation units 204 c and 204 drespectively so as to obtain a predictive image.

In this case, the linear prediction operation unit 204 c firstcalculates the predictive image using the equation 5 based on theweighting coefficient set (sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1,c_(—)1), so it is possible to calculate the predictive image using bitshift operations without using multiplications. The same goes for thelinear prediction operation unit 204 d. On the other hand, in the caseof FIG. 18, since the average of the weighting coefficient set(sign1_(—)1, m_(—)1, sign2_(—)1, n_(—)1, c_(—)1) and the weightingcoefficient set (sign1_(—)2, m_(—)2, sign2_(—)2, n_(—)2, c_(—)2) iscalculated first, the average of m_(—)1 and m_(—)2 or the average ofn_(—)1 and n_(—)2, namely, the exponents of 2, could not be integers,and thus there is a possibility that the processing amount increases.Also, if the exponents of 2 are rounded off to integers, there isanother possibility that the error increases.

Next, the case where a predictive image is generated in the functionalstructure as shown in FIG. 20A will be explained. The linear predictionoperation unit 204 g calculates a predictive image using the equation 5,based on the parameters sign1_(—)1 and m_(—)1 which are obtained fromthe linear prediction coefficient storage unit 206 c and used for bitshift operations; the parameters sign2_(—)2 and n_(—)2 which areobtained from the linear prediction coefficient storage unit 206 d andused for bit shift operations; and the average c calculated by theaverage calculation unit 204 f, of the parameters c_(—)1 and c_(—)2which are obtained from the linear prediction coefficient storage units206 c and 206 d.

In this case, since the coefficients used for bit shift operations arethe values which are obtained directly from the linear predictioncoefficient storage unit 206 c or the linear prediction coefficientstorage unit 206 d, the exponents of 2 in the equation 5 are integers.Therefore, the calculations can be performed using bit shift operations,and thus the processing amount can be reduced.

The case where a predictive image is generated in the functionalstructure as shown in FIG. 20B will be explained. The linear predictionoperation unit 204 h calculates a predictive image using the equation 5based on the parameters sign1_(—)1, m_(—)1 and c_(—)1 which are obtainedfrom the linear prediction coefficient storage unit 206 e and theparameters sign2_(—)2 and n_(—)2 which are obtained from the linearprediction coefficient storage unit 206 f.

In this case, since the coefficients used for bit shift operations arevalues which are obtained directly from the linear predictioncoefficient storage unit 206 e or the linear prediction coefficientstorage unit 206 f, the exponents of 2 in the equation 5 are integers.Therefore, the predictive image can be calculated using bit shiftoperations, and thus the processing amount can be reduced.

In the cases of FIGS. 20A and 20B, there are parameters which do notneed to be added to the coded stream for transmission, as is the casewith the cases of FIGS. 10A and 10B in the sixth embodiment, and thedata amount of the coded stream can be reduced.

Using the linear prediction equations as explained in the seventhembodiment, the calculations can be performed using bit shift operationswithout using multiplications, so the processing amount can besignificantly reduced from that in the sixth embodiment.

In the present embodiment, linear prediction is performed using theequations 5, 6 and 7 instead of the equations 1, 3 and 4, and using theparameter set to be coded (sign1, m, sign2, n, c) instead of (w1, w2, c,d), so the calculations can be realized only using bit shift operationsand thus reduction of processing amount is achieved. However, it is alsopossible, as another approach, to use the equations 1, 3 and 4 and (w1,w2, c, d) as they are, by limiting the selectable values of w1 and w2 toonly the values available for bit shift operations, so the calculationscan be realized only using bit shift operations and thus reduction ofprocessing amount is achieved in exactly the same structure as that inthe sixth embodiment.

In addition, as a method for such limitation, it is possible to limitthe values of w1 and w2 so that 1 is always selected for such values andinput a coded stream having arbitrary values of only c1 and c2 which areDC components. Taking the structure of FIG. 21 as an example, (1,c_(—)1) for ref1 and (1, c_(—)2) for ref2 shall be coded as parametersets. In this case, the pixel value P(i) of the predictive image iscalculated by the following equation in which w1_(—)1 and w2_(—)2 in theequation 1a are substituted for 1.P(i)=(Q1(i)+Q2(i))/pow(2, d)+(c _(—)1+c _(—)2)/2

-   -   (where pow(2, d) indicates the “d”th power of 2)

Accordingly, it is possible to significantly reduce the processingamount for linear prediction even in exactly the same structure as thatof the sixth embodiment.

In addition, as shown in FIG. 24, in the case where a flag sft_flg,indicating whether it is possible or not to perform linear predictionusing only bit shift operations, and a flag dc_flg, indicating whetherit is possible or not to perform linear prediction using only c that isa DC component, are described in picture common information of a codedstream of a picture, the decoding apparatus can perform decoding,referring to these flags, in the structure suitable for linearprediction using only bit shift operations or decoding in the structuresuitable for linear prediction using only a DC component. Accordingly,processing amount can be reduced significantly depending on thestructure of the decoding apparatus.

Eighth Embodiment

The moving picture decoding method in the eighth embodiment of thepresent invention will be explained. Since the structure of the decodingapparatus, the flow of decoding processing and the reference indexassignment method are exactly same as those in the sixth embodiment, theexplanation thereof is not repeated here.

As explained in the background art, there is a method for generating apredictive image using a predetermined fixed equation such as theequation 2a and the equation 2b, unlike the sixth and seventhembodiments in which a predictive image is generated using a predictionequation obtained from weighting coefficient sets of linear predictioncoefficients. This conventional method has an advantage that a dataamount for coding can be reduced because there is no need to code andtransmit the weighting coefficient set used for generating thepredictive image. Also, the processing amount for linear prediction canbe significantly reduced because the equations for linear prediction aresimple. However, this method using the fixed equations has a problemthat prediction accuracy is lowered because there are only twoselectable linear prediction equations 2a and 2b.

So, the equations 8a and 8b, instead of the equations 2a and 2b, areused in the present embodiment. These equations 8a and 8b are obtainedby adding C1 and C2 to the equations 2a and 2b. Since only the number ofadditions increases for the operation, there is little increase inprocessing amount, compared with the original equations 2a and 2b.

The equations 8a and 8b are prediction equations for generating apredictive image with reference to two pictures at a time, but whengenerating a predictive image with reference to only one picture, theequation 9 is used instead of the equation 3 or 4 as explained in theabove embodiments.

The weighting coefficient sets for using this method are only (C1) forref1 and (C2) for ref2. Therefore, an example of a coded stream of apicture obtained using this method is as shown in FIG. 14. In the sliceheader area, the weighting coefficient sets for linear prediction(pset0, pset1, pset2, . . . ) are described separately for ref1 andref2, and each of the weighting coefficient sets includes only C.Similarly, FIG. 15 shows an example of weighting coefficient setsincluded in the slice header area. Differently from FIG. 7, each of theweighting coefficient sets in FIG. 15 includes only C.

FIG. 22 is a block diagram showing the functional structure forgenerating a predictive image via the linear prediction coefficientstorage unit 206 and the motion compensation decoding unit 204 in FIG.2.

A predictive image is generated via the linear prediction coefficientstorage unit 206 a, the linear prediction coefficient storage unit 206 band the linear prediction operation unit 204 a.

The weighting coefficient sets (C1) and (C2) having one elementrespectively are obtained from the linear prediction coefficient storageunits 206 a and 206 b by the first reference index ref1 and the secondreference index ref2 outputted from the coded stream analysis unit 201.These values are inputted to the linear prediction operation unit 204 a,where linear prediction is performed on them using the equations 8a and8b, and then the predictive image is generated.

Similarly, when linear prediction is performed with reference to onlyone picture, either one of the weighting coefficient sets (C1) and (C2)is obtained using only one of ref1 and ref2 in FIG. 22, linearprediction is performed using the equation 9, and then a predictiveimage is generated.

Since the present embodiment requires only one parameter to be used foreach of ref1 and ref2, the data amount to be described in the codedstream can be reduced. Also, since the linear prediction equations donot require complex operations such as multiplications, the operationamount can also be minimized. In addition, the use of the coefficientsC1 and C2 allows dramatic improvement of the low prediction accuracy,which is considered as a disadvantage of the conventional method ofusing fixed equations.

Note that it is possible to use the linear prediction method asexplained in the present embodiment, regardless of whether a pluralityof reference indices can refer to the same picture or not.

Ninth Embodiment

The moving picture decoding method in the ninth embodiment of thepresent invention will be explained. Since the structure of the decodingapparatus, the flow of decoding processing and the reference indexassignment method are exactly the same as the sixth embodiment, theexplanation thereof is not repeated here.

Display order information indicating display time or the alternativethereto as well as a picture number are assigned to each picture. FIG.39 is a diagram showing an example of picture numbers and thecorresponding display order information. Certain values are assigned tothe display order information according to display order. In thisexample, the value which increases one by one for each picture is used.A method for generating values of coefficients used in an equation forlinear prediction using this display order information will be explainedin the ninth embodiment.

In the sixth embodiment, linear prediction is performed for each pixelusing the equation 1, the equation 3 or the equation 4 when generating apredictive image in motion compensation. However, since the linearprediction requires data of coefficients, such coefficient data isdescribed in slice header areas in a coded stream as weightingcoefficient sets for use for creation of the predictive image. Althoughthis method achieves high coding efficiency, it requires additionalprocessing for creating data of the weighting coefficient sets andcauses increase in bit amount because the weighting coefficient sets aredescribed in the coded stream.

So, it is also possible to perform linear prediction using the equation10, equation 11a or the equation 12a instead of the equation 1. Usingthese equations, the weighting coefficients can be determined based ononly the display order information of each reference picture, so thereis no need to code the weighting coefficient sets separately.

When it is assumed, as an example, that the current picture to bedecoded is No. 16, the picture designated by the first reference indexis No. 11, and the picture designated by the second reference index isNo. 10, the display order information of respective pictures are 15, 13and 10, and thus the following linear prediction equations aredetermined.V1=(10-15)/(10-13)=5/3V2=(15-13)/(10-13)=−2/3P(i)=5/3×Q1(i)−⅔×Q2(i)

Compared with the method for performing linear prediction usingweighting coefficient sets in the equation 1, the above equations havelower flexibility of coefficient values, and thus it can be said that itis impossible to create the optimum predictive image. However, comparedwith the method of switching the two fixed equations 2a and 2b dependingon the positional relationship between two reference pictures, the aboveequations are more efficient linear prediction equations.

When both the first reference index and the second reference indexreferto the same picture, the equation 11a and the equation 12a do nothold because T1 becomes equal to T2. So, when the two reference pictureshave the same display order information, linear prediction shall beperformed using ½ as the values of V1 and V2. In that case, the linearprediction equations are as follows.V1=½V2=½P(i)=½×Q1(i)+½×Q2(i)

Also, the equations 11a and 12a do not hold because T1 becomes equal toT2 when the first reference index and the second reference index referto difference pictures but these pictures have the same display orderinformation. When the two reference pictures have the same display orderinformation as mentioned above, linear prediction shall be performedusing ½ as the values of V1 and V2.

As described above, when two reference pictures have the same displayorder information, it is possible to use a predetermined value as acoefficient. Such predetermined coefficient may be one having the sameweight like ½ as shown in the above example.

By the way, use of the equation 10 in the present embodiment requiresmultiplications and divisions for linear prediction. Since the linearprediction operation using the equation 10 is the operation on all thepixels in a current block to be decoded, addition of multiplicationscauses significant increase in the processing amount.

So, approximation of V1 and V2 to the powers of 2 like the case of theseventh embodiment makes it possible to perform linear predictionoperation by only shift operations, and thus to achieve reduction of theprocessing amount. As the linear prediction equations for that case, theequations 11b and 12b are used instead of the equations 11a and 12a.

Note that it is also possible to use the equations 11c and 12c insteadof the equations 11a and 12a.

Note that it is also possible to use the equations 11d and 12d insteadof the equations 11a and 12a.

Note that the value of V1 and V2 approximated to the power of 2 shallbe, taking the equation 11b as an example, the value of ±pow(2, v1)obtained when the values of ±pow(2, v1) and (T2−T0)/(T2−T1) are mostlyapproximated to each other as the value of v1 changes one by one.

For example, in FIG. 39, when a current picture to be decoded is No. 16,the picture designated by the first reference index is No. 11, and thepicture designated by the second reference index is No. 10, the displayorder information of the respective pictures are 15, 13 and 10, so(T2−T0)/(T2−T1) and ±pow(2, v1) are as follows.(T2−T0)/(T2−T1)=(10-15)/(10-13)=5/3+pow(2, 0)=1+pow(2, 1)=2

Since 5/3 is close to 2 rather than 1, V=2 is obtained as a result ofapproximation.

As another method of approximation, it is also possible to switchbetween round-up approximation and round-down approximation depending onthe relationship between two values of display order information T1 andT2.

In that case, round-up approximation is performed on both V1 and V2 whenT1 is later than T2, and round-down approximation is performed on bothV1 and V2 when T1 is earlier than T2. It is also possible, in a reversemanner, to perform round-down approximation on both V1 and V2 when T1 islater than T2, and round-up approximation on both V1 and V2 when T1 isearlier than T2.

As another approximation method using display order information,round-up approximation is performed in an equation for V1 and round-downapproximation is performed in an equation for V2 when T1 is later thanT2. As a result, the difference between the values of the twocoefficients becomes large, so it is likely that values suitable forextrapolation are obtained. On the contrary, when T1 is earlier than T2,the value in the equation for V1 and the value in the equation for V2are compared, and then round-up approximation is performed on thesmaller value and round-down approximation is performed on the largervalue. As a result, the difference between the values of the twocoefficients becomes small, so it is likely that values suitable forinterpolation are obtained.

For example, in FIG. 39, when a current picture to be decoded is No. 16,the picture designated by the first reference index is No. 11, and thepicture designated by the second reference index is No. 10, the displayorder information of the respective pictures are 15, 13 and 10. Since T1is later than T2, round-up approximation is performed on the equationfor V1 and round-down approximation is performed on the equation for V2.As a result, the equations 11b and 12b are calculated as follows.(T2−T0)/(T2−T1)=(10-15)/(10-13)=5/3+pow(2, 0)=1+pow(2, 1)=2  (1) Equation 11b

As a result of round-up approximation, V1=2 is obtained.(T0-T1)/(T2−T1)=(15-13)/(10-13)=−⅔−pow(2, 0)=−1−pow(2, −1)=−½  (2) Equation 12b

As a result of round-down approximation, V2=−1 is obtained.

Note that although the equation 10 is only one equation for linerprediction in the above embodiments, it is also possible to combine thismethod with the linear prediction method by use of two fixed equations2a and 2b as explained in the background art. In that case, the equation10 is used instead of the equation 2a, and the equation 2b is used as itis. To be more specific, the equation 10 is used when the picturedesignated by the first reference index appears behind the picturedesignated by the second reference index in display order, whereas theequation 2b is used in other cases.

It is also possible, on the contrary, to use the equation 10 instead ofthe equation 2b and use the equation 2a as it is. To be more specific,the equation 2 a is used when the picture designated by the firstreference index appears behind the picture designated by the secondreference index, and the equation 10 is used in other cases. However,when the two reference pictures have the same display order information,linear prediction is performed using ½ as the values of V1 and V2.

It is also possible to describe only the coefficient C in the sliceheader areas to be used for linear prediction, in the same manner as theconcept of the eighth embodiment. In that case, the equation 13 is usedinstead of the equation 10. V1 and V2 are obtained in the same manner asthe above embodiments.

The processing for generating coefficients is needed, and further thecoefficient data needs to be decoded into the slice header area, but useof C1 and C2 allows more accurate linear prediction even if the accuracyof V1 and V2 is low. This is effective particularly in the case where V1and V2 are approximated to the powers of 2 for linear prediction.

Note that using the equation 13, the linear prediction can be performedin the same manner in both cases where one reference index is assignedto one picture and a plurality of reference indices are assigned to onepicture.

In the calculation of the values in each of the equations 11a, 12a, 11b,12b, 11c, 12c, 11d and 12d, the combinations of available values arelimited to some extent in every slice. So, only one operation is enoughfor decoding a slice, unlike the equation 10 or the equation 13 in whichthe operation needs to be performed for all the pixels in a currentblock to be decoded, and thus there seems to be little influence on theentire processing amount.

Note that the display order information in the present embodiment is notlimited to display order, but may be the actual display time, or theorder of respective pictures starting from a predetermined picture, withits value increasing as the lapse of display time.

Tenth Embodiment

The moving picture decoding method in the tenth embodiment of thepresent invention will be explained. Since the structure of the decodingapparatus, the flow of decoding processing and the reference indexassignment method are exactly same as the sixth embodiment, theexplanation thereof is not repeated here.

In the conventional method, it is possible to switch, if necessary,between generation of a predictive image by use of fixed equations andgeneration of a predictive image by use of weighting coefficient sets oflinear prediction coefficients, using flags described in a picturecommon information area in a coded stream.

In the present embodiment, another method for switching the variouslinear prediction methods as explained in the above sixth to ninthembodiments using flags will be explained.

FIG. 17A shows the structure used for the case where five flags (p_flag,c_flag, d_flag, t_flag, s_flag) for controlling the above switching aredescribed in the slice header area in the coded stream.

As shown in FIG. 17B, p_flag is a flag indicating whether weightingcoefficients have been coded or not. c_flag is a flag indicating whetheronly the data of the parameter C (C1 and C2), out of the parameters forref1 and ref2, has been coded or not. t_flag is a flag indicatingwhether weighting coefficients for linear prediction are to be generatedor not using the display order information of the reference pictures.And s_flag is a flag indicating whether the weighting coefficients forlinear prediction are to be approximated to the powers of 2 or not forthe calculation by use of shift operations.

Furthermore, d_flag is a flag indicating whether or not to switch twopredetermined fixed equations like the equations 2a and 2b depending onthe temporal positional relationship between the picture designated byref1 and the picture designated by ref2, when linear prediction isperformed using such two fixed equations. To be more specific, when thisflag indicates the switching of the equations, the equation 2a is usedin the case where the picture designated by ref1 is later than thepicture designated by ref2 in display order and the equation 2b is usedin other cases for performing linear prediction, as is the case with theconventional method. On the other hand, when this flag indicates noswitching of the equations, the equation 2b is always used for linearprediction, regardless of the positional relationship between thepicture designated by ref1 and the picture designated by ref2.

Note that even if the equation 2a is used instead of the equation 2b asan equation to be used without switching, the equation 2a can be handledin the same manner as the equation 2b.

In the decoding apparatus as shown in FIG. 2, the coded stream analysisunit 201 analyzes the value of the flag p_flag, and outputs, to themotion compensation decoding unit 204, the instruction indicatingwhether or not to decode the data concerning the weighting coefficientsets so as to generate a predictive image based on the result of theanalysis, and then the motion compensation decoding unit 204 performsmotion compensation by linear prediction. As a result, it is possible touse the weighting coefficient sets in a higher performance apparatus forperforming linear prediction, and not to use the weighting coefficientsets in a lower performance apparatus for performing linear prediction.

Similarly, in the decoding apparatus as shown in FIG. 2, the codedstream analysis unit 201 analyzes the value of the flag c_flag, andoutputs, to the motion compensation decoding unit 204, the instructionindicating whether or not to decode only the data concerning theparameter C (C1 and C2) corresponding to the DC components of image dataso as to generate a predictive image by use of fixed equations, based onthe result of the analysis, and then the motion compensation decodingunit 204 performs motion compensation by linear prediction. As a result,it is possible to use the weighting coefficient sets in a higherperformance apparatus for performing linear prediction, and to use onlythe DC components in a lower performance apparatus for performing linearprediction.

Similarly, in the decoding apparatus as shown in FIG. 2, when the codedstream analysis unit 201 analyzes the value of the flag d_flag andlinear prediction is performed using fixed equations based on the resultof the analysis, the coded stream analysis unit 201 outputs theinstruction indicating whether or not to switch two equations fordecoding to the motion compensation decoding unit 204, in which motioncompensation is performed. As a result, it is possible to switch themethod, such that either one of the fixed equations is used for linearprediction in the case where there is little temporal change inbrightness of a picture, and the two fixed equations are switched forlinear prediction in the case where the brightness of the picturechanges as time goes by.

Similarly, in the decoding apparatus as shown in FIG. 2, the codedstream analysis unit 201 analyzes the value of the flag t_flag and basedon the result of the analysis, it outputs, to the motion compensationdecoding unit 204, the instruction indicating whether or not to generatethe coefficients for linear prediction using the display orderinformation of the reference pictures, and the motion compensationdecoding unit 204 performs motion compensation. As a result, it ispossible for the coding apparatus to code the weighting coefficient setsfor linear prediction in the case where more coding can be done, and togenerate the coefficients from the display order information for linearprediction in the case where no more coding can be done.

Similarly, in the coding apparatus as shown in FIG. 2, the coded streamanalysis unit 201 analyzes the value of the flag s_flag, and based onthe result of the analysis, it outputs, to the motion compensationdecoding unit 204, the instruction indicating whether or not toapproximate the coefficients for linear prediction to the powers of 2 soas to allow the calculation of these coefficients by shift operations,and the motion compensation decoding unit 204 performs motioncompensation. As a result, it is possible to use the weightingcoefficients without being approximated in a higher performanceapparatus for performing linear prediction, and to use the weightingcoefficients with being approximated to the powers of 2 in a lowerperformance apparatus for performing linear prediction which can berealized by shift operations.

For example, the case of (1) (p, c, d, t, s_flag)=(1, 0, 0, 0, 1) showsthat all the weighting coefficient sets are decoded and linearprediction is performed by only shift operations by representing thecoefficients as the powers of 2 as explained in the seventh embodiment,so as to generate a predictive image.

The case of (2) (p, c, d, t, s_flag)=(1, 1, 1, 0, 0) shows that only thedata concerning the parameter C (C1 and C2) is decoded, the method forgenerating a predictive image by adding the coefficient C to the fixedequations as explained in the eighth embodiment is used, and further thetwo fixed equations are switched for use.

In the case of (3) (p, c, d, t, s_flag)=(0, 0, 0, 0, 0), no weightingcoefficient set is decoded. In other words, it shows that the method forgenerating a predictive image using only the equation 2b, out of thefixed equations as the conventional method, is used.

The case of (4) (p, c, d, t, s_flag)=(0, 0, 1, 1, 1) shows that noweighting coefficient set is decoded, but linear prediction is performedby only shift operations by generating the weighting coefficients fromthe display order information of the reference pictures and furtherapproximating the coefficients to the powers of 2, as explained in theninth embodiment, and then the two fixed equations are switched for use,so as to generate a predictive image.

Note that in the present embodiment, the determinations are made usingfive flags (p_flag, c_flag, d_flag, t_flag, s_flag), each of which is 1bit, but it is also possible to represent the determinations with onlyone flag of 5 bits, instead of these five flags. In that case, decodingby use of variable length decoding is available, not by representationwith 5 bits.

Note that in the present embodiment, all the five flags (p_flag, c_flag,d_flag, t_flag, s_flag), each of which is 1 bit, are used, but the samecan be applied to the case where linear prediction method is switchedonly using some of these flags. In that case, the flags necessary forthe linear prediction are only coded and described, out of the flags asshown in FIG. 17A.

In the conventional method, a flag is provided in a picture commoninformation area of a coded stream for switching between generation of apredictive image by use of fixed equations and generation of apredictive image by use of weighting coefficient sets of linearprediction coefficients, so as to allow the switching between them on apicture-by-picture basis. However, in this method, the predictive imagegeneration method can only be switched on a picture-by-picture basis.

On the contrary, in the present embodiment as mentioned above, it ispossible to switch the method for generating a predictive image for eachslice that is a subdivision of a picture by providing the switching flagin a slice header of a coded stream. Therefore, it is possible, forexample, to generate the predictive image by use of the weightingcoefficient sets in a slice containing complex images, whereas togenerate the predictive image by use of the fixed equations in a slicecontaining simple images. As a result, the image quality can be improvedwhile minimizing the increase in processing amount.

Note that in the present embodiment, five flags (p_flag, c_flag, d_flag,t_flag, s_flag) are described in a slice header area for thedetermination of the method in each slice, but it is also possible toswitch the determination on a picture-by-picture basis by describingthese flags in the picture common information area. In addition, it isalso possible to generate a predictive image by the optimum method on ablock-by-block basis by providing the switching flag on each block beinga subdivision of a slice.

Note that the display order information in the present embodiment is notlimited to display order, but may be the actual display time, or theorder of respective pictures starting from a predetermined picture, withits value increasing as the lapse of display time.

Eleventh Embodiment

The moving picture coding method and the moving picture decoding methodin the eleventh embodiment of the present invention will be explained.Since the structures of the coding apparatus and the decoding apparatus,the flows of coding processing and decoding processing, and thereference index assignment method are exactly the same as those in thefirst and sixth embodiments, the explanation thereof is not repeatedhere.

In the present embodiment, the technology which is similar to thatexplained in the fifth and tenth embodiments will be explained.

The flag p_flag indicating whether a parameter set is coded or not andthe flag c_flag indicating whether only the data concerning theparameter C (C1 and C2) is coded or not, out of the parameters for ref1and ref2, are described in each slice.

In the coding apparatus as shown in FIG. 1, the motion compensationcoding unit 107 determines, on a slice-by-slice basis or ablock-by-block basis, whether or not to code the data concerning theparameter set, and outputs, based on the determination, the informationof the flag p_flag to the coded stream generation unit 103, in whichthat information is described into the coded stream, as shown in FIG.40A.

Similarly, in the coded apparatus as shown in FIG. 1, the motioncompensation coding unit 107 determines, on a slice-by-slice basis or ablock-by-block basis, whether or not to code only the data concerningthe parameter C (C1 and C2) corresponding to the DC components of imagedata, and outputs, based on the determination, the information of theflag c_flag to the coded stream generation unit 103, in which theinformation is described into the coded stream, as shown in FIG. 40A.

On the other hand, in the decoding apparatus as shown in FIG. 2, thecoded stream analysis unit 201 analyzes the values of the switchingflags p_flag and c_flag, and based on the analysis, outputs, to themotion compensation decoding unit 204, the instruction indicatingwhether to generate a predictive image by use of the downloadedparameter sets, or to generate a predictive image by use of fixedequations, for example, and the motion compensation decoding unit 204performs motion compensation by linear prediction.

For example, as shown in FIG. 40B, (1) when the flag p_flag is 1 and theflag c_flag is 0, the coding apparatus codes all the parameter sets, (2)when the flag p_flag is 1 and the flag c_flag is 1, the coding apparatuscodes only the data concerning the parameter C (C1 and C2), and further(3) when the flag p_flag is 0 and the flag c_flag is 0, the codingapparatus does not code any parameter sets. Note that by determining theflag values as shown in FIG. 40B, it can be found whether the DCcomponent of the image data has been coded or not using the value of theflag p_flag.

The coding apparatus processes the parameters as explained in FIG. 8 toFIG. 10, for example, in the above case (1). It processes the parametersas explained in FIG. 16, for example, in the above case (2). Itprocesses the parameters using fixed equations, for example, in theabove case (3).

The decoding apparatus processes the parameters as explained in FIG. 18to FIG. 20, for example, in the above case (1). It processes theparameters as explained in FIG. 22, for example, in the above case (2).It processes the parameters using fixed equations, for example, in theabove case (3).

Another example of a combination of the above cases (1) to (3) will beexplained specifically.

In the above example, coding of parameters (whether or not to have thedecoding apparatus receive the parameters) is switched explicitly usingthe flags p_flag and c_flag, but it is also possible to use a variablelength coding table (VLC table) instead of the above flags.

As shown in FIGS. 41A and 41B, it is also possible to select explicitlywhether to switch between the fixed equation 2a and the fixed equation2b.

Here, no switching of the equation 2 means the following. It isexplained in the background art, for example, that, in order to generatea predictive image, the fixed equation 2a including fixed coefficientsis selected when the picture designated by the first reference indexappears behind in display order the picture designated by the secondreference index, and the equation 2b including fixed coefficients isselected in other cases. On the other hands, when it is instructed, asshown in the example of FIG. 41B, not to switch the equation, it meansthat the fixed equation 2b including fixed coefficients is selected evenwhen the picture designated by the first reference index appears behindin coding order the picture designated by the second reference index soas to generate a predictive image.

The information of the flag v_flag for selecting explicitly whether toswitch between the fixed equation 2a and the fixed equation 2b isoutputted from the coded stream generation unit 103 and described in thecoded stream as shown in FIG. 41A.

FIG. 41B shows an example of processing by use of the flag v_flag. Asshown in FIG. 41B, when the flag v_flag is 1, the parameters are notcoded (the parameters are not downloaded on the decoding apparatus. Thesame goes for the following.), and the fixed equation 2 is not switched.When the flag v_flag is 01, the parameters are not coded, and the fixedequation 2 is switched. When the flag v_flag is 0000, only the parameterC is coded, and the equation 2 is not switched.

Further, when the flag v_flag is 0001, only the parameter C is coded,and the equation 2 is switched. When the flag v_flag is 0010, all theparameters are coded, and the equation 2 is not switched. When the flagv_flag is 0011, all the parameters are coded, and the fixed equation 2is switched.

Note that since all the parameters are coded when v_flag is 0010 and0011, it is possible to perform linear prediction using weightingparameters, without using fixed equations, and in that case, thedetermination of whether to switch the fixed equation or not for use isignored.

Note that the flag v_flag can be switched by the motion compensationcoding unit 107 in the coding apparatus as shown in FIG. 1 and themotion compensation decoding unit 204 in the decoding apparatus as shownin FIG. 2. It is also possible to use the flag d_flag indicating whetherto switch the fixed equation or not, instead of the flag v_flag, inaddition to the above flags p_flag and c_flag.

As described above, use of flags allows switching of whether or not thedecoding apparatus receives (downloads) coded parameters after thecoding apparatus codes the parameters. As a result, the parameters to becoded (or received) can be switched explicitly according to thecharacteristics of the application and the performance of the decodingapparatus.

In addition, since the fixed equation can be switched explicitly, thevariety of means for improving the image quality is increased, and thusthe coding efficiency is also improved. Also, even if the decodingapparatus does not have a fixed equation, it can switch to the fixedequation explicitly, and thus generate a predictive image using theexplicitly selected fixed equation.

Note that the placement of the flags is not limited to that as shown inFIG. 40. Also, the values of the flags are not limited to those asexplained above. Furthermore, since four types of uses of parameters canbe explicitly shown using two types of flags, the parameters may beassigned in the manners other than those as explained above. Inaddition, all the parameters are transmitted in the above example, butall the necessary parameter sets may be transmitted, as explained inFIG. 10 and FIG. 20, for example.

Twelfth Embodiment

If a program for realizing the structures of the picture coding methodor the picture decoding method as shown in each of the above embodimentsis recorded on a memory medium such as a flexible disk, it becomespossible to perform the processing as shown in each of the embodimentseasily in an independent computer system.

FIGS. 25A, 25B and 25C are illustrations showing the case where theprocessing is performed in a computer system using a flexible disk whichstores the picture coding method or the picture decoding method of theabove first to eleventh embodiments.

FIG. 25B shows a front view and a cross-sectional view of an appearanceof a flexible disk, and the flexible disk itself, and FIG. 25A shows anexample of a physical format of a flexible disk as a recording mediumbody. The flexible disk FD is contained in a case F, and a plurality oftracks Tr are formed concentrically on the surface of the disk in theradius direction from the periphery and each track is divided into 16sectors Se in the angular direction. Therefore, as for the flexible diskstoring the above-mentioned program, the picture coding method as theprogram is recorded in an area allocated for it on the flexible disk FD.

FIG. 25C shows the structure for recording and reproducing the programon and from the flexible disk FD. When the program is recorded on theflexible disk FD, the picture coding method or the picture decodingmethod as a program is written in the flexible disk from the computersystem Cs via a flexible disk drive. When the picture coding method isconstructed in the computer system by the program on the flexible disk,the program is read out from the flexible disk using the flexible diskdrive and transferred to the computer system.

The above explanation is made on the assumption that a recording mediumis a flexible disk, but the same processing can also be performed usingan optical disk. In addition, the recording medium is not limited to aflexible disk and an optical disk, but any other medium such as an ICcard and a ROM cassette capable of recording a program can be used.

Thirteenth Embodiment

FIG. 26˜FIG. 29 are illustrations of devices for performing the codingprocessing or the decoding processing as described in the aboveembodiments and a system using them.

FIG. 26 is a block diagram showing the overall configuration of acontent supply system ex100 for realizing content distribution service.The area for providing communication service is divided into cells ofdesired size, and base stations ex107 to ex110 which are fixed wirelessstations are placed in respective cells.

In this content supply system ex100, devices such as a computer ex111l,a personal digital assistant (PDA) ex112, a camera ex113, a mobile phoneex114 and a camera-equipped mobile phone ex115 are connected to theInternet ex 101 via an Internet service provider ex102, a telephonenetwork ex104 and base stations ex107 to ex110.

However, the content supply system ex100 is not limited to theconfiguration as shown in FIG. 26, and a combination of any of them maybe connected. Also, each device may be connected directly to thetelephone network ex104, not through the base stations ex107 to ex110.

The camera ex113 is a device such as a digital video camera capable ofshooting moving pictures. The mobile phone may be a mobile phone of apersonal digital communications (PDC) system, a code division multipleaccess (CDMA) system, a wideband-code division multiple access (W-CDMA)system or a global system for mobile communications (GSM) system, apersonal handyphone system (PHS), or the like.

A streaming server ex103 is connected to the camera ex113 via the basestation ex109 and the telephone network ex104, which allows livedistribution or the like using the camera ex113 based on the coded datatransmitted from a user. Either the camera ex113 or the server fortransmitting the data may code the data.

Also, the moving picture data shot by a camera ex116 may be transmittedto the streaming server ex103 via the computer ex111. The camera ex116is a device such as a digital camera capable of shooting still andmoving pictures. Either the camera ex116 or the computer ex111 may codethe moving picture data. An LSI ex117 included in the computer ex111 orthe camera ex116 actually performs coding processing.

Software for coding and decoding moving pictures may be integrated intoany type of a storage medium (such as a CD-ROM, a flexible disk and ahard disk) being a recording medium which is readable by the computerexll or the like. Furthermore, a camera-equipped mobile phone ex115 maytransmit the moving picture data. This moving picture data is the datacoded by the LSI included in the mobile phone ex115.

The content supply system ex100 codes contents (such as video of livemusic performance) shot by users using the camera ex113, the cameraex116 or the like in the same manner as the above embodiments andtransmits them to the streaming server ex103, while the streaming serverex103 makes stream distribution of the content data to the clients attheir request. The clients include the computer ex111, the PDA ex112,the camera ex113, the mobile phone ex114 and so on capable of decodingthe above-mentioned coded data. In the content supply system ex100, theclients can thus receive and reproduce the coded data, and further theclients can receive, decode and reproduce the data in real time so as torealize personal broadcasting.

When each device in this system performs coding or decoding, the movingpicture coding apparatus or the moving picture decoding apparatus, asshown in each of the above-mentioned embodiments, can be used.

A mobile phone will be explained as an example of the device.

FIG. 27 is a diagram showing the mobile phone ex115 that uses the movingpicture coding method and the moving picture decoding method explainedin the above embodiments. The mobile phone ex115 has an antenna ex201for sending and receiving radio waves to and from the base stationex110, a camera unit ex203 such as a CCD camera capable of shootingvideo and still pictures, a display unit ex202 such as a liquid crystaldisplay for displaying the data obtained by decoding video and the likeshot by the camera unit ex203 and received via the antenna ex201, a bodyunit including a set of operation keys ex204, a voice output unit ex208such as a speaker for outputting voices, a voice input unit 205 such asa microphone for inputting voices, a storage medium ex207 for storingcoded or decoded data such as data of moving or still pictures shot bythe camera, and text data and data of moving or still pictures ofreceived e-mails, and a slot unit ex206 for attaching the storage mediumex207 to the mobile phone ex115. The storage medium ex207 includes aflash memory element, a kind of an electrically erasable andprogrammable read only memory (EEPROM) that is an electrically erasableand rewritable nonvolatile memory, in a plastic case such as an SD card.

The mobile phone ex115 will be further explained with reference to FIG.28. In the mobile phone ex115, a power supply circuit unit ex310, anoperation input control unit ex304, a picture coding unit ex312, acamera interface unit ex303, a liquid crystal display (LCD) control unitex302, a picture decoding unit ex309, a multiplex/demultiplex unitex308, a record/reproduce unit ex307, a modem circuit unit ex3O6 and avoice processing unit ex305 are connected to a main control unit ex311structured for overall controlling the display unit ex202 and the bodyunit including the operation keys ex204, and they are connected to eachother via a synchronous bus ex313.

When a call-end key or a power key is turned ON by a user's operation,the power supply circuit unit ex310 supplies respective units with powerfrom a battery pack so as to activate the camera-equipped digital mobilephone ex115 for making it into a ready state.

In the mobile phone ex115, the voice processing unit ex305 converts thevoice signals received by the voice input unit ex205 in conversationmode into digital voice data under the control of the main control unitex311 including a CPU, ROM, RAM and others, the modem circuit unit ex306performs spread spectrum processing of the digital voice data, and thesend/receive circuit unit ex301 performs digital-to-analog conversionand frequency transform of the data, so as to transmit it via theantenna ex201. Also, in the mobile phone ex115, after the data receivedby the antenna ex201 in conversation mode is amplified and performed offrequency transform and analog-to-digital conversion, the modem circuitunit ex306 performs inverse spread spectrum processing of the data, andthe voice processing unit ex305 converts it into analog voice data, soas to output it via the voice output unit 208.

Furthermore, when transmitting e-mail in data communication mode, thetext data of the e-mail inputted by operating the operation keys ex204on the body unit is sent out to the main control unit ex311 via theoperation input control unit ex304. In the main control unit ex311,after the modem circuit unit ex306 performs spread spectrum processingof the text data and the send/receive circuit unit ex301 performsdigital-to-analog conversion and frequency transform for it, the data istransmitted to the base station ex110 via the antenna ex201.

When picture data is transmitted in data communication mode, the picturedata shot by the camera unit ex203 is supplied to the picture codingunit ex312 via the camera interface unit ex303. When the picture data isnot transmitted, it is also possible to display the picture data shot bythe camera unit ex203 directly on the display unit 202 via the camerainterface unit ex303 and the LCD control unit ex302.

The picture coding unit ex312, which includes the picture codingapparatus as explained in the present invention, compresses and codesthe picture data supplied from the camera unit ex203 by the codingmethod used for the picture coding apparatus as shown in the aboveembodiments so as to transform it into coded picture data, and sends itout to the multiplex/demultiplex unit ex308. At this time, the mobilephone ex115 sends out the voices received by the voice input unit ex205during shooting by the camera unit ex203 to the multiplex/demultiplexunit ex308 as digital voice data via the voice processing unit ex305.

The multiplex/demultiplex unit ex308 multiplexes the coded picture datasupplied from the picture coding unit ex312 and the voice data suppliedfrom the voice processing unit ex305 by a predetermined method, themodem circuit unit ex306 performs spread spectrum processing on themultiplexed data obtained as a result of the multiplexing, and thesend/receive circuit unit ex301 performs digital-to-analog conversionand frequency transform on the data for transmitting via the antennaex201.

As for receiving data of a moving picture file which is linked to a Webpage or the like in data communication mode, the modem circuit unitex306 performs inverse spread spectrum processing of the signal receivedfrom the base station ex110 via the antenna ex201, and sends out themultiplexed data obtained as a result of the processing to themultiplex/demultiplex unit ex308.

In order to decode the multiplexed data received via the antenna ex201,the multiplex/demultiplex unit ex308 separates the multiplexed data intoa coded bit stream of picture data and a coded bit stream of voice data,and supplies the coded picture data to the picture decoding unit ex309and the voice data to the voice processing unit ex305 respectively viathe synchronous bus ex313.

Next, the picture decoding unit ex309, which includes the picturedecoding apparatus as explained in the present invention, decodes thecoded bit stream of picture data by the decoding method corresponding tothe coding method as shown in the above-mentioned embodiments so as togenerate reproduced moving picture data, and supplies this data to thedisplay unit ex202 via the LCD control unit ex302, and thus movingpicture data included in a moving picture file linked to a Web page, forinstance, is displayed. At the same time, the voice processing unitex305 converts the voice data into analog voice data, and then suppliesthis data to the voice output unit ex208, and thus voice data includedin a moving picture file linked to a Web page, for instance, isreproduced.

The present invention is not limited to the above-mentioned system, andat least either the picture coding apparatus or the picture decodingapparatus in the above-mentioned embodiments can be incorporated into asystem for digital broadcasting as shown in FIG. 29. Such ground-basedor satellite digital broadcasting has been in the news lately. Morespecifically, a coded bit stream of video information is transmittedfrom a broadcast station ex409 to a communication or broadcastingsatellite ex410 via radio waves. Upon receipt of it, the broadcastingsatellite ex410 transmits radio waves for broadcasting, a home-useantenna ex406 with a satellite broadcast reception function receives theradio waves, and a television (receiver) ex401 or a set top box (STB)ex407 decodes the coded bit stream for reproduction.

The picture decoding apparatus as shown in the above-mentionedembodiments can be implemented in the reproduction apparatus ex403 forreading off and decoding the coded bit stream recorded on a storagemedium ex402 that is a recording medium such as a CD and DVD. In thiscase, the reproduced video signals are displayed on a monitor ex404. Itis also conceived to implement the picture decoding apparatus in the settop box ex407 connected to a cable ex405 for a cable television or theantenna ex406 for satellite and/or ground-based broadcasting so as toreproduce them on a monitor ex408 of the television ex401. The picturedecoding apparatus may be incorporated into the television, not in theset top box. Or, a car ex412 having an antenna ex411 can receive signalsfrom the satellite ex410, the base station ex107 or the like forreproducing moving pictures on a display device such as a car navigationsystem ex413.

Furthermore, the picture coding apparatus as shown in theabove-mentioned embodiments can code picture signals for recording on arecording medium. As a concrete example, there is a recorder ex420 suchas a DVD recorder for recording picture signals on a DVD disc ex421 anda disk recorder for recording them on a hard disk. They can also berecorded on an SD card ex422. If the recorder ex420 includes the picturedecoding apparatus as shown in the above-mentioned embodiments, thepicture signals recorded on the DVD disc ex421 or the SD card ex422 canbe reproduced for display on the monitor ex408.

As the structure of the car navigation system ex413, the structurewithout the camera unit ex203, the camera interface unit ex303 and thepicture coding unit ex312, out of the units shown in FIG. 28, can beconceived. The same applies to the computer ex111, the television(receiver) ex401 and others.

In addition, three types of implementations can be conceived for aterminal such as the above-mentioned mobile phone ex114: asending/receiving terminal including both an encoder and a decoder; asending terminal including an encoder only; and a receiving terminalincluding a decoder only.

As described above, it is possible to use the moving picture codingmethod or the moving picture decoding method in the above-mentionedembodiments in any of the above-mentioned apparatuses and systems, andusing this method, the effects described in the above embodiments can beobtained.

INDUSTRIAL APPLICABILITY

The present invention is suitable for a picture coding apparatus forperforming inter-picture coding to generate a predictive image bygenerating commands indicating correspondence between picture numbersand reference indices for designating reference pictures andcoefficients used for generation of predictive images, designating areference picture to be referred to using a reference index, whenperforming motion compensation on a block in a current picture to becoded, and performing linear prediction using a coefficientcorresponding to the reference index on a block obtained by motionestimation in the designated reference picture. In addition, the presentinvention is suitable for a picture decoding apparatus for decoding acoded signal obtained as a result of the coding by the picture codingapparatus.

1. A picture decoding method comprising: decoding a coded image signalto obtain: a reference index that identifies a reference picture for acurrent block to be decoded; commands that indicate correspondencebetween reference indices and picture numbers indicating a decodingorder of coded pictures; information that indicates a maximum referenceindex value; and a prediction error; designating, based on the commandsand the reference index, a reference picture which is referred to whenthe current block is decoded through motion compensation; generating aweighting coefficient based on display order information that indicatesa display order of pictures; generating a predictive image by performinglinear prediction, using the weighting coefficient, on pixel values of areference block obtained from the reference picture designated in saiddesignating; and generating a reconstructed image from the predictiveimage and the prediction error, wherein the reference index includes afirst reference index that identifies a first reference picture and asecond reference index that identifies a second reference picture, andin said generating of the predictive image, the predictive image isgenerated by performing linear prediction on the pixel values of thereference block using a pre-defined weighting coefficient when the firstreference picture identified by the first reference index and the secondreference picture identified by the second reference index haveidentical display order information.
 2. The picture decoding methodaccording to claim 1, wherein the pre-defined weighting coefficient forthe first reference picture is identical to the pre-defined weightingcoefficient for the second reference picture.
 3. A picture decodingapparatus comprising: a coded image signal decoding unit operable todecode a coded image signal to obtain: a reference index that identifiesa reference picture for a current block to be decoded; commands thatindicate correspondence between reference indices and picture numbersindicating a decoding order of coded pictures; information thatindicates a maximum reference index value; and a prediction error; areference picture designating unit operable to designate, based on thecommands and the reference index, a reference picture which is referredto when the current block is decoded through motion compensation; aweighting coefficient generating unit operable to generate a weightingcoefficient based on display order information that indicates a displayorder of pictures; a predictive image generating unit operable togenerate a predictive image by performing linear prediction, using theweighting coefficient, on pixel values of a reference block obtainedfrom the reference picture designated by said reference picturedesignating unit; and a reconstructed image generating unit operable togenerate a reconstructed image from the predictive image and theprediction error, wherein the reference index includes a first referenceindex that identifies a first reference picture and a second referenceindex that identifies a second reference picture, and in said predictiveimage generating unit, the predictive image is generated by performinglinear prediction on the pixel values of the reference block using apre-defined weighting coefficient when the first reference pictureidentified by the first reference index and the second reference pictureidentified by the second reference index have identical display orderinformation.
 4. A computer-readable data storage medium on which aprogram for decoding a coded image signal is stored, wherein the programcauses a computer to perform the decoding by the picture decoding methodaccording to claim
 1. 5. A computer-readable data storage medium onwhich a program for decoding a coded image signal is stored, wherein theprogram causes a computer to perform the decoding by the picturedecoding method according to claim 2.